Ukraine has banned entry to its territory for all Russian males aged between 16-60, the Russian flagship carrier Aeroflot said Thursday in a warning to passengers.

"In line with an official order received by the company, all Russian male citizens aged from 16-60 will be refused entry into Ukraine," Aeroflot said, adding that exceptions would only be made in extreme cases.

Russian President Vladimir Putin on Thursday accused NATO chief Anders Fogh Rasmussen of secretly recording and then leaking to the media a private conversation between them when Rasmussen was Danish prime minister.

As NATO secretary general, Rasmussen has used hawkish rhetoric calling for Moscow to "de-escalate" the crisis in Ukraine and pull back thousands of troops massed on the border.

Hundreds of pro-Russian protesters staged a rally in the the eastern city of Mariupol on Thursday after Ukrainian soldiers killed three people during a raid on their barracks.

The three assailants were killed and 13 others wounded when around 300 people attacked the interior ministry troops in the industrial port city overnight with firearms and petrol bombs, according to Interior Minister Arsen Avakov.

The European Union said it had agreed Thursday to hold talks with Russia on its gas supplies to Europe through Ukraine, warning Moscow its reliability as an energy source was at stake.

The EU "agrees on your proposal for consultations with the Russian Federation and Ukraine with regard to security of gas supply and transit," European Commission head Jose Manuel Barroso said in reply to an April 10 letter from President Vladimir Putin.

French President Francois Hollande warned Thursday that EU sanctions on Russia could be strengthened if there is no progress at talks in Geneva on the Ukraine crisis.

"We could raise the level of sanctions if no solution emerges (in Geneva), but that is not our desire," Hollande said at a joint press conference with Czech Prime Minister Bohuslav Sobotka.

Japan's foreign minister has postponed a visit to Russia due to "scheduling issues", the ministry said Thursday, as Tokyo finds itself in a delicate position over the crisis in Ukraine.

The ministry did not give a new date for a visit by Fumio Kishida, who had been due to attend a now-canceled intergovernmental committee on trade and economic issues with first deputy prime minister Igor Shuvalov.

Russia and Ukraine sat down Thursday for Western-backed talks on the escalating crisis in the former Soviet republic as Russian President Vladimir Putin accused the authorities in Kiev of dragging the country towards the abyss.

In a dramatic worsening of tensions in the restive east, three pro-Moscow separatists were killed in an overnight gunbattle with Ukrainian troops in the southeastern port city of Mariupol.

The United States warned Wednesday it was "actively preparing" new sanctions to hit Russia if critical Ukraine talks do not produce concessions from Moscow.

U.S. officials privately signaled they had little hope that the Geneva talks between Russia, Ukraine, the European Union and Washington would make significant progress.

Ukraine will demand that Russia cease its support for "terrorist activities" on its territory, Kiev's foreign minister said Wednesday on the eve of crunch talks with Moscow, the U.S. and EU.

"Our main demand is to de-escalate the situation in eastern Ukraine. We want Russia to withdraw the troops from the eastern borders of Ukraine. We want Russia not to support terrorist activities in eastern Ukraine," Foreign Minister Andriy Deshchytsya said after he landed in Geneva for Thursday's meeting.

The United States warned Russia Wednesday to stop its "provocation" in eastern Ukraine, reiterating that new sanctions could be slapped on the Kremlin.

With U.S. Secretary of State John Kerry en route to Europe for international talks on the most serious East-West crisis in years, and a Ukrainian effort to reassert control over its eastern regions floundering, the State Department demanded de-escalation and demobilization on the part of Russian forces.