The challenge came only hours after an unexpected diplomatic breakthrough late Thursday worked out between Kiev, Moscow, Washington and Brussels after talks in Geneva to defuse the deepest East-West crisis since the end of 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 increase 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 blared 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 also remained defiant, holed up inside a seized police stationwith six armoured personnel carriers captured from the Ukrainian army on Wednesday still parked on the streets.

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

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.

– ‘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.

“I don’t think we can be sure of anything at this point,” Obama said after the talks.

Ukrainian Prime Minister Arseniy Yatsenyuk told parliament on Friday he also had “no high hopes” of the agreement holding.

Obama warned that if there was no progress in days, “additional consequences” would be imposed on Russia on top of sanctions the United States and European Union have already imposed on 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.

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.”

No timeline was given for the implementation of the agreement.

Ukraine’s Foreign Minister Andriy Deshchytsya said he would meet in Kiev with representatives from the Organisation for Security and Cooperation in Europe (OSCE) who are tasked with verifying the accord is taking effect.