NATO has said it is taking immediate steps to boost its military presence in eastern Europe in response to Russian "aggression" in Ukraine.

Its Secretary-General, Anders Fogh-Rasmussen, announced the 28-nation alliance - which does not include Ukraine - had agreed on a package of further military measures to reinforce its members' defences.

It also agreed to deploy more air and sea forces, as well as increase the readiness of land forces, to the Baltic and eastern Mediterranean.

Mr. Rasmussen said Moscow must make clear "it doesn't support the violent actions of well-armed militias or pro-Russian separatists" in eastern Ukraine.

The move came as pro-Russian militias took two Ukrainian soldiers hostage, according to the AFP news agency, as tensions in the region continued to rise.

Ukraine's Defence Minister said the two men had been seized "by extremists" near the town of Krasnyi Luch in Lugansk province and taken to an unknown destination.

Its security service said it had intercepted communications showing Russian military commanders in eastern Ukraine had issued "shoot-to-kill orders" against Ukrainian soldiers.

'Building a new Berlin Wall'

Ukraine's Prime Minister Arseniy Yatsenyuk accused Russia of building "a new Berlin Wall" in a return to the Cold War era - and "exporting terrorism" to his country.

"All of these terrorist operations by the Russian special services are not just unacceptable, they are an international crime" he said.

Armoured vehicles flying Russian flags entered the eastern Ukrainian town of Slavyansk and were cheered by some locals.

Reporter Katie Stallard, at the scene, said "There was a crowd of a few hundred people here earlier who were greeting these people as heroes".

In Donetsk, 20 masked pro-Russian gunmen reportedly took control of the mayor's office.

Earlier, the British Foreign Secretary William Hague warned Russia was in danger of making a "grave miscalculation" if it continued with its campaign to destabilise Ukraine.

Kiev is trying to regain control of a number of government buildings in the east of the country which have been taken over by pro-Russia separatists.

Last night the military successfully took back an airfield in Kramatorsk from around 30 armed militants. It was the first military intervention since the launch of an "anti-terrorist operation" by Kiev, announced by interim president Oleksandr Turchynov.

However, Ukraine's Defence Ministry said six armoured personnel carriers had been seized in Kramatorsk by pro-Russia separatists - and were now in Slavyansk.

The rising tensions come ahead of talks between Ukraine, Russia, the United States and the European Union on Thursday in Geneva.