Ukrainian troops have attacked pro-Russian separatists in the southern port of Mariupol, apparently driving them out of buildings they had occupied in the centre of the city.
Mariupol is the second-largest city in the eastern Donetsk region that has declared independence from the government in Kiev. The key port sits along the main road leading from Russia to the Crimean peninsula, which Russia annexed in March from Ukraine.
About 100 soldiers emerged from the previously rebel-occupied buildings, shouting the name of their battalion, Azov, and singing the Ukrainian national anthem. They also destroyed an armoured vehicle and a heavy truck used by the separatists, leaving the vehicles scorched and riddled with large-calibre bullet holes.
Interior minister Arsen Avakov said four government troops were wounded in what he called a successful operation. Witnesses said they saw troops capture at least four separatist fighters. There was no immediate word of casualties on the rebel side.
Ukraine and the West have accused Moscow of fomenting the unrest in eastern Ukraine and supporting the separatist fighters, but Russia has denied sending troops or weapons to Ukraine and has described the Russian citizens fighting with the separatists as volunteers.
The renewed fighting came as rebel leaders confirmed they now have three tanks.
Government officials said the tanks were part of a column of armoured vehicles that crossed the porous border into Ukraine from Russia, but there has been no independent confirmation that they came from Russia.
Denis Pushilin, a leader of the separatist Donetsk People's Republic, told Russian state television that they have the tanks but it was "improper to ask" where they had come from.
"They are in Donetsk and are the minimum that we have to defend the city," he said.
The US State Department and Nato have said if the tanks did come from Russia, it would be a "serious escalation" of the crisis.
"I am concerned about reports that pro-Russian armed gangs are acquiring heavy weapons from Russia, including Russian tanks," Nato secretary-general Anders Fogh Rasmussen said in a statement.
"We have seen reports that Russian tanks and other armoured vehicles may have crossed the border into eastern Ukraine. If these reports are confirmed, this would mark a serious escalation of the crisis in eastern Ukraine."
Mr Rasmussen urged Moscow "to complete the withdrawal of its military forces on the border with Ukraine, to stop the flow of weapons and fighters across the border, and to exercise its influence among armed separatists to lay down their weapons and renounce violence".
There was no immediate response from Russia.
Mr Pushilin repeated the separatists' call for Russia to send peacekeeping troops into eastern Ukraine. Moscow has said this could only be done with UN authorisation.
Ukrainian president Petro Poroshenko, who took office on June 7, yesterday rallied support for his plan to end the fighting in phone calls with Russian president Vladimir Putin and German chancellor Angela Merkel.
Mr Poroshenko told Ms Merkel he is willing to negotiate, but not with those he calls terrorists. He said he could offer amnesty only to people who don't have "blood on their hands".
According to his spokesman, Svyatoslav Tsegolko, Mr Poroshenko told Mr Putin it was "unacceptable" that tanks had crossed the border.