The moves came after the European Union signed broad trade-and-political agreements with Ukraine, Moldova and Georgia, pushing the bloc's influence eastward but potentially deepening strains with Russian President Vladimir Putin.
The pacts, which lower trade barriers and aim to promote democratic reforms, were years in the making but faced doubts recently as Moscow sought to reassert its influence in the former Soviet republics.
It was former Ukrainian President Viktor Yanukovych's decision not to sign the EU agreement in November that sparked the huge pro-Western protests that eventually led to his ouster.
That in turn sparked unrest in parts of eastern Ukraine including the Crimean peninsula, which Russia quickly annexed.
Ukraine's new President Petro Poroshenko announced a weeklong cease-fire on June 20, a step that was to lead to talks on his plan to end the insurrection.
After signing the trade pact Friday in Brussels, he said it would be impossible for Ukraine to maintain the cease-fire much longer if the pro-Russia separatists, as well as Moscow, continued to ignore his peace push and launch attacks.
Early Saturday, after conferring with his security advisers back in Kiev, Mr. Poroshenko announced that he had decided to extend the cease-fire by 72 hours, to Monday night.
By then, Mr. Poroshenko said on the presidential website that he expected the four steps spelled out earlier in Brussels by European leaders for avoiding new sanctions would be met.
They include establishing "effective control of the border" with independent monitors, the release of hostages and the launch of peace talks.
"Ukraine reserves the right to suspend the cease-fire early if there is any use of force or a failure to follow these conditions," the presidential statement said.
The White House later endorsed the four steps.
"Failing to take them only increases the likelihood that additional economic costs will be imposed" on Russia, press secretary Josh Earnest told reporters traveling with President Barack Obama on Air Force One.
The EU leaders said they would assess the situation next week and would impose fresh sanctions "should it be required."
That is likely to include adding names to the current list of individual Russian or separatist officials slapped with asset freezes and visa bans, EU officials said.
Russian firms could also be targeted, on top of the two Crimea-based companies already listed.
The leaders warned they were prepared to come back "at any time" and impose broader sanctions on Russia if the situation doesn't improve.
Mr. Earnest said the U.S. wasn't prepared to draw a "clear line" between meeting the four demands and imposing new sanctions, but added that the U.S. has shown "a clear willingness to act in concert with our partners and allies to further isolate Russia."
Several officials said the Ukrainian leader's arguments played a key role in shaping the EU sanctions threat, which was much tougher than had been expected.
"Until now Russia [is] doing nothing" to help the cease-fire, Mr. Poroshenko said in Brussels.
"Can you imagine? We declare a cease-fire and they send tanks."
Ukraine and the West have accused Russia of sending heavy weaponry and fighters across the porous border to join the rebels—a charge Moscow has denied.
Earlier Friday, Mr. Putin called for a "long term cease-fire as a necessary condition for holding in-depth talks."
Rebel leader Alexander Borodai, after meeting with government representatives Friday, said his group was ready to extend the cease-fire, but wouldn't agree to cede control of border posts they have seized.
He also said that the talks with Kiev would be possible only if Ukraine withdraws all security forces from the Donetsk and Luhansk regions, where the fighting has been concentrated.
Friday's talks followed another night of clashes in the Donetsk region, with the government saying militants killed five soldiers and wounded several more.
Each side has accused the other of instigating violence.
However, the separatists did free four observers from the Organization for Security and Cooperation in Europe on Friday after holding them for more than a month.
A second team of observers captured on May 29 in eastern Ukraine hasn't been released, although Mr. Borodai promised the observers would be soon.
Meanwhile, the United Nations refugee agency said that 110,000 Ukrainians have fled to Russia to escape the fighting, with tens of thousands more having been internally displaced.
In Brussels, Mr. Poroshenko said he had "a full understanding that this war is impossible to win just by military means."
"We should fight for the hearts and the brains of the people" in the east, he said.
He accused the separatists of being under Moscow's "direct control."
By signing the agreements with Georgia, Ukraine and Moldova, EU leaders hoped to show they won't let a newly aggressive Russia deter them from welcoming countries into the European orbit.
The leaders of the three countries said the agreements are a pivotal step in aligning their countries permanently with Europe.
But the Kremlin has made it clear it sees the deals as a threat to its rightful sphere of influence.
Russia's Foreign Ministry reiterated Friday that the signing of the pacts will have serious consequences for those countries' relations with Russia, although it didn't say what.
The deals are part of an eastward-looking EU strategy launched in 2009, with an EU-Ukraine agreement always the centerpiece.
Azerbaijan, Belarus and Armenia turned down similar bilateral pacts.
EU leaders have insisted that efforts to strengthen ties with their eastern neighbors weren't intended to isolate Moscow.
"There is nothing in these agreements, nor in the European Union's approach, that might harm Russia in any way," European Council President Herman Van Rompuy said Friday.
Russia fought a war against Georgia in 2008, and still occupies the Georgian regions of Abkhazia and South Ossetia.
In Moldova, Russia has significant influence over the separatist region of Transnistria, where it has stationed thousands of troops.
In Washington, the State Department offered congratulations to Georgia, Moldova and Ukraine for signing the agreements, saying that they mark "a major step toward integrating these Eastern Partnership countries more closely with the European Union and realizing a Europe whole, free, and at peace."
Source: Wall Street Journal