Poland's prime minister said Wednesday there is evidence that regular units of the Russian army are operating in Ukraine, according to information from NATO and Polish intelligence.

"No one can take seriously talk of separatists in Ukraine at this point," Prime Minister Donald Tusk told parliament regarding the crisis in neighboring Ukraine.

Russia's foreign minister said Wednesday that Moscow was not interested in breaking up Ukraine despite respecting declarations of independence by the two main eastern regions.

"We expressed full respect for the results (of the May referendums) and said the political implementation of what the citizens of southeast Ukraine voted for must be realized through negotiations. We are not interested in breaking up (Ukraine)," Sergei Lavrov said during a meeting with Kremlin supporters in central Russia.

NATO is preparing a rapid response to the Ukraine crisis that would allow it to deploy troops swiftly to reinforce eastern Europe and dissuade Russia from further destabilizing the region, the alliance chief Anders Fogh Rasmussen said on Wednesday.

Russia on Wednesday denied a claim by Finland that it held secret talks with the United States in June on ways to resolve the Ukraine conflict.

Kiev on Wednesday said a massive convoy of tanks and heavy weapons from Russia was travelling towards a government-held town in restive east Ukraine.

Rebels in east Ukraine appeared on Wednesday to have seized swathes of territory from government forces who abandoned vehicles and ammunition as they retreated, while Western intelligence said Russian army units were operating inside the country.

After weeks of successful military operations that have seen government forces push deep into the last rebel bastions, the tide appears to be turning once again in the four-month conflict, prompting a nervous government in Kiev to call on NATO for help.

The presidents of Russia and Ukraine on Tuesday sat down to crunch one-on-one talks over heavy fighting in east Ukraine as the Kremlin admitted for the first time its troops had entered its neighbor's territory.

Ukrainian President Petro Poroshenko and Russian leader Vladimir Putin began a key bilateral meeting after six hours of difficult negotiations alongside top EU officials in Minsk aimed at defusing a crisis some fear could lead to all-out war between Kiev and Moscow.

The presidents of Russia and Ukraine hold key talks Tuesday with little hopes for a breakthrough in resolving the raging conflict pitting Kiev against pro-Moscow separatist rebels.

Hours before the crunch talks, Kiev ratcheted up tensions by releasing footage purporting to show 10 Russian soldiers captured on its territory who a Moscow military source claimed had crossed into Ukraine "by accident."

Hungary's Prime Minister Viktor Orban on Monday said he would seek to form a political alliance to stop the European Union pulling away from Russia over the crisis in Ukraine.

"The EU gets further away from Russia every day. That's not only bad for Hungary, but for the entire EU," Orban told Hungarian ambassadors in Budapest.

The Kremlin announced plans on Monday for a second aid convoy into Ukraine, heightening tensions on the eve of crunch talks, while fresh clashes raged along the border and around the main rebel-held city.

A Ukrainian military spokesman said troops were battling a contingent of "10 tanks and two armored personal carriers" flying rebel flags that had crossed a southern part of the border with Russia and was heading towards the government-held city of Mariupol.