DONETSK, Ukraine -- Military momentum seemed to tilt in Ukraine’s favor on Friday with government forces winning control of the port of Mariupol, the second largest city in the separatist region of Donetsk and Russia being on the defensive about allegations it had allowed three tanks to cross the border in support of hard-pressed, pro-Russian militias.
The Ukrainian Army reported that it had destroyed two of the three tanks and several other vehicles in the convoy that the authorities in Kiev said had crossed from Russia on Thursday.
The apparent border crossing brought a rebuke from Anders Fogh Rasmussen, the secretary general of the North Atlantic Treaty Organization, who said that, if confirmed, it would “mark a serious escalation of the crisis in eastern Ukraine.”
The spokesman for Russian President Vladimir V. Putin, Dmitry S. Peskov, responded by saying a Ukrainian armored personnel carrier had crossed into Russia on Friday, for unclear reasons, and was abandoned there.
The Russian state news agency reported that the Russian border service said the occupants had then fled back to Ukraine.
That disputed crossing was said to have occurred in the Luhansk region, where separatists control the border.
The Russian statement did not clarify whether pro-Russian militants or Ukrainian soldiers were in the vehicle.
The Ukrainian assault in Mariupol left five pro-Russian militants dead and four Ukrainian soldiers wounded, and ended with a Ukrainian flag’s being hoisted at City Hall as the military routed the last separatists from the city’s administration buildings.
In Friday’s confrontation, videos posted online showed Ukrainian soldiers standing over captives who were lying face down with hands clenched behind their heads.
The videos also showed soldiers displaying trophies of the battle — captured orange-and-black ribbons and shoulder patches of a pro-Russian group, the Russian Orthodox Army — and speaking with prisoners freed from the occupied buildings.
In that pro-Ukrainian forces — volunteer patrols of factory workers, a Ukrainian nationalist battalion called Azov and the Ukrainian military — had controlled most of Mariupol for weeks, the action was as much a propaganda victory as a military one.
In fighting near the besieged town of Slavyansk, a Grad ground-to-ground rocket killed one person and wounded several others.
Residents said the rocket went off course and hit a vegetable market in the village of Dobroyepole.
The Ukrainian military blamed separatists, though most shelling originated on the Ukrainian side.
Two months of fighting have turned the green flatlands of the border region into a jumbled quilt of pro-Russian and Ukrainian-controlled cities and regions, with near continual low-level combat in contested zones.
Over the long term, Ukraine, with its far larger though badly equipped and poorly trained army, has more forces, and it is unclear how long the separatists can hold out without more support from Russia.
That is something Moscow cannot offer openly without risking more severe Western sanctions.
The result is misdirection and sleight of hand, and a conflict of endless puzzles and mind games.
The daytime journey of the three tanks through eastern Ukraine, for example, which was filmed in multiple videos and witnessed by Western reporters, could not have been more obvious, and yet the convoy was too small to serve a military purpose.
Was it a warning?
Their appearance followed the Ukrainian military’s gains of recent days and Russian objections to continuing action, even as cease-fire talks were underway between Russia, Ukraine and the Organization for Security and Cooperation in Europe.
A rebel leader, Denis Pushilin, told Russian state television Friday that the separatists had tanks but that it was “improper to ask” where they had gotten them.
President Petro O. Poroshenko of Ukraine called Mr. Putin on Thursday to protest that Moscow had allowed the tanks and other armored vehicles to cross a remote and poorly guarded border into Ukraine from Russia, Mr. Poroshenko’s spokesman said.
Though the tanks were irrefutably in Ukraine, where they had come from was less clear; rebels escorting them told Reuters they had been taken from a Ukrainian military base.
Russia ratcheted up the economic pressure on Ukraine on Friday in their dispute over natural gas supplies and prices.
Gazprom’s chief executive, Aleksei B. Miller, said that the company would not negotiate further with Ukraine over gas prices until Kiev paid its debts for past deliveries, and would adhere to a Monday deadline demanding prepayment for future supplies.
Gazprom, the Russian state-controlled energy giant, is demanding $385 for a thousand cubic meters of gas; Ukraine’s natural gas company, Naftogaz, has said that it will pay the debt if Gazprom accepts an 18-month contract at a price of $326.
Ukraine’s prime minister, Arseny Yatsenyuk, ordered Naftogaz to prepare for a Russian gas cutoff Monday and to consider raising the transit tariff charged Gazprom to ship gas across Ukraine to Europe.
Source: The New York Times