While Kiev made significant advances against rebels in the country's east in recent days, Ukrainian and U.S. officials say Russian weapons are continuing to pour over the border.
The escalation in fighting suggests Russian President Vladimir Putin has no intention of dialing back his support for the separatists, denting Western hopes that international attention from the airliner crash would force him to change course.
"The fact that you have two additional planes shot down speaks to the pattern we've seen over the last several weeks—which is Russian-backed separatists, armed with Russian anti-aircraft [weapons], posing risks to aircraft in Ukraine," said Ben Rhodes, a deputy White House national security adviser.
Kiev has pressed on with its offensive against the rebels despite calls to halt the fighting in the aftermath of the crash.
The government said Wednesday that Kremlin-backed separatists in the east were attacking Ukrainian troops with Russian-supplied truck-mounted missile launchers, mining buildings, and blowing up bridges and burning down wheat fields upon their retreat.
Kiev said it retook two cities even as a days-long fight continued for Lysychansk, the base of a leading insurgent commander.
Putin, who has denied supporting the rebels, remained defiant.
His apparent unwillingness to pressure the separatists to lay down their arms—even after the global outcry over the downing of Malaysia Airlines Flight 17 that killed 298 civilians—poses a challenge for U.S. and European diplomats who have for months tried to offer him a diplomatic route to step back from Ukraine.
The shoot-down of the Ukrainian warplanes—an attack the rebels took credit for but that Kiev said it believed came from Russian territory—marks the first time a plane has been brought down over Ukraine since Flight 17 was downed last Thursday.
The U.S. is working with Ukraine to determine the circumstances surrounding the downing of the Su-25 fighters, Col. Steve Warren, the Pentagon press secretary, said.
U.S. officials said they had developed little information on the most recent strikes and their origin as of Wednesday.
But officials have said "a preponderance of the evidence" indicates that a Ukrainian cargo plane brought down July 14 was hit by a surface-to air missile fired from the Russian side of the border.
With Putin appearing undeterred from continuing to fuel a conflict in Ukraine's east in what diplomats and analysts say is an attempt to cripple Kiev's turn toward the West, senior European diplomats will meet Thursday to decide on new sanctions targets.
They will also discuss a plan to impose sanctions on entire sectors of the Russian economy, including high-tech goods and oil and gas exploration equipment.
Foreign ministers from the European Union this week said they would activate that plan if Russia didn't use its sway over the rebels to allow international investigators access to the Malaysia Airlines crash site and stop the flow of weapons and men across the border from Russia.
With progress being made on the first condition, an EU diplomat said governments will be focusing on whether Putin has scaled back his alleged support for the rebels.
"We're pleased with the steps they're taking," Mr. Rhodes said, noting European countries are considering possible sanctions in energy, finance and arms sectors.
"The message they sent yesterday is that they're looking into sectorial sanctions in these areas, and that is certainly what we want to see from them."
A decision to impose sanctions targeting entire Industrial sectors could come as early as next week, European diplomats have said.
While Kiev says it has made significant advances against the rebels in recent days—and nearly halved the territory they hold in the past five weeks—the continued Kremlin support for the insurgents raises the prospect of a bloody and drawn-out conflict.
The rebels say they are retreating in order to dig in around a smaller number of towns, which are closer to the border with Russia that Ukraine's army says it hasn't been able to seal.
Russia has continued its support to the separatists even since the crash of Flight 17, sending tanks and rocket launchers, U.S. intelligence officials said.
"We don't think they have stopped," said one.
"We think they continue to do it."
Moscow denies this.
Russia and the West have both called for talks, but Ukrainian officials say the rebels are little more than bands of gunmen whose main leaders are Russian citizens with few ties to the region.
That makes it hard for Kiev to negotiate over its "peace plan," which offers greater local powers over economic and cultural decisions.
Kiev has previously said that missiles have been fired at its jets from across the border, a claim Russia has denied.
The U.S. and Ukraine have presented a case that Flight 17 was brought down last week in a missile strike fired from a sophisticated Buk missile system they say was provided by Russia and was then smuggled back across the porous border.
Russia has routinely denied aiding the insurgency.
On Wednesday, the two Ukrainian warplanes were flying at an altitude of about 17,000 feet when they were hit, said Ukrainian security spokesman Col. Andriy Lysenko —too high for the rebels to reach with the weapons they are now believed to possess.
"According to our preliminary information, they [the planes] were downed from abroad," Col. Lysenko said.
That account couldn't be independently confirmed.
A message posted on a social-media page associated with rebel military leader Igor Girkin, who goes by the nom de guerre Igor Strelkov, claimed responsibility for destroying the two planes using shoulder-fired, surface-to-air missiles.
Video posted on the site showed the remains of what appeared to be the planes smoldering in grassy fields in rebel-held territory.
The shoulder-fired missiles have a much lower reach and wouldn't be able to hit a plane at 17,000 feet.
Neither Russia nor the rebels immediately responded to Col. Lysenko's claim.
Moscow continued to come under rhetorical fire from the West, a day after the European Union agreed to escalate sanctions.
"Russia has in the past promised much to defuse the conflict, but has rarely delivered," a German Foreign Ministry spokeswoman said.
A spokesman for German Chancellor Angela Merkel said that reports continued to come in of Russian weapons streaming across the border and that the Kremlin didn't appear to be interested in a comprehensive investigation of the causes of the Malaysia Airlines crash.
He said the Kremlin also didn't appear to be willing to fully exercise its influence on the rebels, who are led, the spokesman said, by longtime Russian intelligence agents.
Mr. Putin has shown few signs of backing down.
On Monday, Russian defense officials presented an alternate version of events in the Flight 17 crash, suggesting the Malaysian airliner had been possibly shot down by the Ukrainian government, a theory viewed with deep skepticism in the West.
The following day Mr. Putin offered blistering criticism of Ukraine's continued military operation against the separatists, and accused the West of trying to weaken Russia and of issuing ultimatums.
The Defense Ministry said the two Su-25 planes were brought down near the villages of Savur-Mohyla and Dmytrivka, which sit close to the Russian border and are around 20 kilometers (12 miles) from Torez, where the civilian jet was brought down a week ago.
The pilots managed to successfully eject from the planes, the ministry said.
The Ukrainian defense ministry said the two attack planes had been providing aerial support to ground units fighting with the rebels along the Russian border.
Earlier on Wednesday, Strelkov told Russia's LifeNews website that rebel fighters had abandoned two nearby key positions along the border at Kozhevnya and Chervona Zorya to avoid becoming surrounded.
He said rebel fighters had pulled back from the town of Karlivka, just west of the regional capital of Donetsk, for the same reason.
Col. Lysenko said Ukrainian forces had also recaptured the cities of Severodonetsk and Popasna in the Luhansk region and were fighting for the nearby city of Lysychansk.
He said that the flow of arms across the Russian border appeared to be continuing and said Russian forces were also exerting "psychological pressure" on Ukrainian border guards by charging toward them in tanks as though they were about to attack and then turning away at the last minute.
He said retreating separatists were also destroying infrastructure, such as a bridge near Horlivka.
Ukraine's State Border Guard Service said several of its posts had come under mortar and rocket fire overnight Wednesday, claiming the bombardment had come from the Russian side of the border.
Two days before Flight 17 crashed, Ukraine said one of its AN-26 cargo planes was shot down from an altitude of 21,000 feet by a missile near the Russia border.
It said the missile appeared to have come from Russia, although Moscow denied any responsibility.
The next day, Ukraine's defense ministry said an Su-25 attack plane was shot down in Donetsk by a Russian military plane.
Moscow again, as usual, denied any involvement.
Source: The Wall Street Journal