Chechnya's Moscow-backed leader insists he has not sent troops to Ukraine to fight alongside pro-Russia separatists.
In a statement, Ramzan Kadyrov said that as part of the Russian Federation, Chechnya had no armed forces - and that any Chechens operating in Ukraine were there in a personal capacity.
Monday saw some of the worst fighting since rebels seized much of the east.
The separatists say they lost up to 100 fighters as they tried to seize Donetsk airport from pro-Kiev forces.
Ukraine's interior ministry says the military is now in full control of the airport, although gunfire was reported in Donetsk itself on Wednesday.
A government fighter jet was seen flying over the city.
New President Petro Poroshenko has vowed to tackle the eastern uprising. "We will no longer let these terrorists kidnap people and kill them," he told Germany's Bild newspaper in an interview published on Wednesday.
Many of the separatists involved in the clashes at the airport were reported to be part of a unit called the Vostok (East) Battalion, said to include fighters from the northern Caucasus.
In a statement released on Wednesday, Mr Kadyrov said: "Ukrainian sources have been circulating reports that some Chechen units from Russia have invaded Donetsk. I officially declare that this is not true."
He added: "There are three million Chechens and two-thirds of them live outside the Chechen Republic, including in the West. We cannot know and are not supposed to know which of them goes where."
Meanwhile, the Organization for Security and Co-operation in Europe says it has lost contact with a monitoring team. Four of its monitors were on a routine mission east of Donetsk when they were stopped at a checkpoint late on Monday, the security body says.
A spokesman told the BBC the men were Turkish, Swiss, Estonian and Danish. Danish Trade Minister Mogens Jensen said it was believed they were being held by armed separatists.
In a separate development, Poland's foreign ministry says a Polish Roman Catholic priest abducted in Donetsk on Tuesday has been released.
Mr Poroshenko won an outright majority in a presidential election on Sunday.
The poll was called after President Viktor Yanukovych was deposed in February, amid mass protests against his pro-Russian policies.
Separatists in Donetsk and Luhansk declared independence after referendums on 11 May, a move not recognised by Kiev or its Western allies.
The two regions took their cue from a disputed referendum in Crimea, which led to Russia's annexation of the southern peninsula.
Kiev and the West accuse Russia of stoking separatist sentiment in eastern Ukraine - a claim President Vladimir Putin denies.
His foreign policy adviser Yury Ushakov reaffirmed on Wednesday that Moscow "respected" the will of Ukraine's voters but also denounced the Ukrainian army's "provocative military actions".