Ukrainian artillery destroyed a "significant" part of a Russian armored column that crossed into Ukraine during the night, President Petro Poroshenko told British Prime Minister David Cameron on Friday, according to Ukraine's presidential website.
Separately, a Ukrainian military spokesman said Ukrainian forces had tracked the Russian armored column as soon as it crossed onto Ukrainian soil. The announcement rattled global markets.
"Appropriate actions were undertaken and a part of it no longer exists," military spokesman Andriy Lysenko told journalists.
Poroshenko's website said he and Cameron had discussed the situation by telephone following British news reports that a Russian armored column had entered Ukrainian territory.
"The president said that this information was reliable and confirmed because a significant part of this equipment had been destroyed in the night by Ukrainian artillery," it said.
Lysenko said the situation in the conflict zone was becoming increasingly tense, with Ukrainian forces, who are fighting pro-Russian separatists, also coming under artillery attack from Russian territory.
Russia, meanwhile, denied that any military vehicles had even entered Ukrainian territory, and accused Ukraine of attempting to disrupt its humanitarian aid mission to eastern Ukraine and called for a ceasefire in the region to allow for the deliveries. The Kremlin has continuously denied sending weapons and troops into Ukraine.
"We draw attention to the sharp intensification of military action by Ukrainian forces with the apparent aim to stop the path, agreed on with Kiev, of a humanitarian convoy across the Russia-Ukraine border," the Foreign Ministry said in a statement.
Ukraine accused that humanitarian convoy of attempting to ferry Russian military personnel on Friday, but Russian Defense Minister Sergei Shoigu told the U.S. that none were present with the aid, the Pentagon said. U.S. Defense Secretary Chuck Hagel had sought clarification about the Russian convoy from the defense minister during a phone call on Friday and was "guaranteed" it did not include Russian military personnel and would not be used as a pretext for intervening in Ukraine, the Pentagon said in a statement.
Russian news wires reported on Friday that Russian Gen. Maj. Igor Konashenkov said no Russian military convoy has crossed the border despite Ukraine's claims. NATO reported the Russian incursion.
It is unclear whether the fighting is focused on an armored vehicle convoy or the aid shipments.
Ukrainian Foreign Minister Pavlo Klimkin said he will meet with his Russian counterpart, Sergey Lavrov, along with foreign ministers from Germany and France in Berlin on Sunday. The Kremlin said the Ukrainian and Russian chiefs of presidential staff met in Russia on Friday.
Still, the reports of a rebuffed incursion may spark further escalation.