Pro-Russian rebels stubbornly refused to cede control of a string of towns in eastern Ukraine on Friday, jeopardising a deal backed by the West and Russia meant to ease tensions and rejecting promises of concessions from Kiev.

The refusal to budge came despite an unexpected diplomatic breakthrough Thursday worked out between Kiev, Moscow, Washington and Brussels following talks in Geneva to defuse the deepest East-West crisis since the Cold War.

If the hard-won agreement collapses -- as Western and Ukrainian leaders fear it might -- the United States has said it will quickly ramp up sanctions on Russia, which it holds responsible for supporting the separatists and stoking the crisis.

The surprise agreement hammered out in Geneva called for "all illegal armed groups" to disarm and leave seized state buildings and squares.

In return they would benefit from an amnesty for actions over the past two weeks that brought Ukraine to the brink of civil war.

But in the main eastern city of Donetsk, where rebel gunmen wearing ski-masks still occupied a barricaded government building, there was defiance, with the Russian national anthem blaring out through speakers.

Denis Pushilin, a prominent member of the self-declared Donetsk Republic, said he agreed that the buildings should be vacated, but that the leaders in Kiev must also leave the buildings "that they are occupying illegally since their coup d'etat" in February.

In nearby Slavyansk, insurgents remained defiant, holed up inside a seized police station.

Unyielding pro-Russian militants also manned barricades of tyres and sandbags in another town of Kramatorsk while Ukrainian military helicopters continued to land at a nearby aerodrome that remains under Kiev's control.

Despite earlier saying that he held out little hope for the peace deal, in a bid to help defuse the pro-Moscow protests Prime Minister Arseniy Yatsenyuk pledged safeguards for the Russian language and a broad decentralisation of power; both key demands of the Russian-speaking militants.

"We will accord special status to the Russian language and guarantee to protect it," Yatsenyuk said.

Meanwhile, Ukraine's Foreign Minister Andriy Deshchytsya suggested the authorities would hold off temporarily from any more military action on the ground to give the agreement time to take effect.

"If this does not begin in the next few days then I think after Easter there will be more concrete actions," Deshchytsya told journalists. Easter in Orthodox Ukraine ends on Monday.

Deshchytsya said he was meeting with representatives from the Organisation for Security and Cooperation in Europe (OSCE) to hammer out a timetable for their representatives to head to eastern Ukraine to verify if the accord is being implemented.

The Ukrainian government and many Western states strongly believe the occupations took place with the active support of elite Russian military units, allegations Moscow denies.

To back their claims, Ukraine said it was holding 10 Russian "spies" it believes were sent to stir up unrest. It has also, according to the Russian airline Aeroflot, banned the entry of Russian males aged 16 to 60.

In a rare success, the Ukrainian army said that it had recovered two of six armoured vehicles captured by separatists during a disastrous military operation earlier in the week.

- 'No high hopes' -

In Kiev, pro-Western protesters who have maintained street barricades since forcing the ouster of pro-Kremlin president Viktor Yanukovych two months ago vowed to stay put.

"For us, for Ukraine, for the people on Maidan, it means nothing, it is a piece of paper. It is an agreement that was signed behind our backs," engineer Valery Levchunets, 46, told AFP on Independence Square.

Leaders also cautioned the deal could fail with US President Barack Obama, who has locked horns with Russian President Vladimir Putin over Ukraine's future, expressing doubts that Moscow will deliver.

Obama warned that if there was no progress in days, "additional consequences" would be imposed on Russia on top of US and EU sanctions already targeting Putin's inner circle.

UN chief Ban Ki-moon said the situation "remains extremely volatile" and he now expected all sides to "show their serious intention" to implement the agreement.

- Putin claims 'right' to invade -

Although Russian diplomats have embraced the Geneva agreement, Putin himself has given no immediate reaction.

On Thursday, the Russian leader warned that Ukraine was plunging into the "abyss" -- and he hoped he would not have to resort to his "right" to send in tens of thousands of troops massed on Ukraine's border.

He blames the turmoil on Kiev's interim leaders, whose authority he does not recognise, and denies that Russian special forces are operating to support the eastern insurgency.

Some analysts say that the agreement, if it works, could be a face-saving solution for Russia.

"I can see why they (Russians) did this as they felt that the sanctions were quite close to being imposed, so they had to take a step back," Kiev-based political scientist Andreas Umland said.

"But I'm sceptical... I don't think it's all over."

Anatoliy Gritsenko, a former Ukrainian defence minister who is a candidate for next month's presidential election, wrote on his website: "Will the Geneva agreement calm the situation in Ukraine? My response is no. This accord doesn't talk about Ukraine's territorial integrity, nor demand that Russia lift its occupation of Crimea."