Ukraine says border guards have attacked a column of armored vehicles it says entered its territory from Russia, as Moscow announced its intention to send a second aid convoy into the country.
Ukrainian security spokesman Leonid Matyukhin said a column of "several dozen tanks and armored vehicles" crossed the border near the southeastern port city of Mariupol on August 25.
"The convoy has been stopped by border guards," Matyukhin said. "The battle is ongoing."
The clashes are reportedly taking place near the town of Novoazovsk, on the Azov Sea, about 10 kilometers from the Russian border.
Semen Semenchenko, the commander of the pro-government Azov militia, said on his Facebook page that the column's movement toward Novoazovsk had been accompanied by artillery shelling from across the border.
He said around 50 armored vehicles had crossed the border and that 40 of them were moving toward Mariupol.
"The invasion of the Russian occupiers is taking place," he said.
A Ukrainian military spokesman was quoted as saying that "Russian forces in the guise of Donbas fighters" have opened a new front in eastern Ukraine.
He said the main highway between Novoazovsk and Mariupol was still under the control of Ukrainian forces.
Until now, fighting in eastern Ukraine has been concentrated around the two big rebel-held cities of Donetsk and Luhansk, north of Mariupol.
Meanwhile, Russian Foreign Minister Sergei Lavrov told reporters on August 25 that Moscow planned to send a second humanitarian aid convoy into Ukraine and hoped for Kyiv's swift cooperation.
Russia drew condemnation from Kyiv and the West when it sent a huge convoy of trucks into eastern Ukraine last week without the government's consent or a Red Cross escort.
In other comments, Lavrov also said he "didn't see anything close to what could be considered humiliating" in publicly parading Ukrainian prisoners of wars through the streets of Donetsk over the weekend.
Correspondents say people watching the August 24 incident in Donetsk shouted "Fascists!" and threw garbage and empty bottles at the prisoners, who walked with their heads bowed.
Most of the prisoners were unshaven and disheveled. They were dressed either in combat fatigues or civilian clothes.
The move was widely criticized by rights organizations as a violation of the Geneva Conventions.
"Concerning the degrading treatment of war prisoners," Lavrov said, "let the lawyers handle it."
Martin Schaefer, a spokesman for the German Foreign Ministry, condemned the move on August 25 as "quite probably" a war crime.
"It is completely distasteful and it's just not done," Schaefer said.
Lavrov also said that Moscow was willing to use any form of diplomacy to end the conflict in Ukraine, "as long as there is a result."
His comments came a day ahead of planned talks on the Ukraine crisis in Minsk involving Russian President Vladimir Putin and Ukrainian President Petro Poroshenko. Officials from the European Commission will also be in attendance.
According to the United Nations, more than 2,000 people have been killed and about 340,000 forced to flee their homes as a result of the conflict.