Pro-Russian separatists on Monday ignored an ultimatum to leave occupied government buildings in eastern Ukraine while another group of rebels attacked a police headquarters as a threatened military offensive by government forces failed to materialise.

>> Crisis-hit Ukraine eyes national status referendum

Rebels in the town of Slaviansk, which was expected on Monday to be the focus of a broad government "anti-terrorist" operation involving the army, issued a bold call for Russian President Vladimir Putin to help them.

Though he said on Monday that the offensive was going ahead, Ukraine's interim president Oleksander Turchinov sacked the state security chief in charge of the operation, signalling possible discord behind the scenes.

Turchinov also took a risky step to try to undercut rebels' demands, by holding out the prospect of a referendum on the future shape of the Ukrainian state. He suggested this could be held at the same time as a presidential election on May 25.

The European Union threatened Russia with more sanctions over its actions in eastern Ukraine, which Britain said was being destabilised by Moscow, although some EU states said diplomacy should be given more time.

As the 9 a.m. (0600 GMT) deadline issued by authorities in Kiev expired, a Reuters reporter in the flashpoint city of Slaviansk, where armed men had seized two government buildings, saw nothing to show the rebels were obeying the ultimatum.

But one of the rebel leaders, in an appeal issued through journalists, asked Putin to "help us as much as you can".

Also in Slaviansk, about 150 km (90 miles) from the Russian border, a small airfield which was occupied by Ukrainian air force planes on Sunday was empty on Monday and pro-separatist forces said they were now in control of it.

In the city of Horlivka, meanwhile, about 100 pro-Russian separatists attacked the police headquarters, a witness told Reuters. Video footage on Ukrainian television showed an ambulance treating people apparently injured in the attack.

In all, separatists have seized government buildings and security facilities in 10 cities.

Angered by the death of a state security officer and the wounding of two comrades near Slaviansk, Turchinov warned rebels that an anti-terrorist operation involving the army would begin unless they laid down their arms, and that the Donbass region of eastern Ukraine "will soon be stabilised".

His warning raised fears of possible Russian military intervention like that in Crimea which Moscow finally annexed from Ukraine last month.

Outside the city council offices stood a group of about 12 armed men in matching camouflage fatigues, with black balaclavas, one of whom was holding a Russian flag.

They said they were Cossacks but did not say where from. One told Reuters: "The borders between Ukraine, Russia and Belarus are artificial and we are here to take them away."

The new element in the operation announced by Turchinov is the inclusion of the army which has not been involved in more than four months of turmoil and is untested in dealing with internal disorder.

The plan to bring in the army shows a lack of confidence in the 30,000-strong interior ministry troops who have been discredited by identification with the leadership of ousted president Viktor Yanukovich.

The defence ministry on Monday remained silent and would not comment on the army's role.

RUSSIAN SPEAKING

Turchinov and other leaders blame Russia, which annexed Crimea when Moscow-backed Yanukovich fled after months of pro-Western protests, for backing a rash of rebellions in Slaviansk and other Russian-speaking towns in eastern Ukraine.

Turchinov said the Kiev leadership was "not against" a nationwide referendum being held on the type of state Ukraine should be and added he was sure it would confirm the wish of the majority for a united, independent country.

A referendum has been one of the demands of the separatist rebels in eastern Ukraine, although they envisage a poll being held only in their home region. Turchinov said such a referendum would have to be approved by parliament.

But some commentators fear the holding of another poll on the same day as the presidential election could be sabotaged in some way by Russia, which does not recognise the legitimacy of the new leadership and is seen in Kiev as undermining moves to normalise the situation in the country.

Russian Foreign Minister Sergei Lavrov said Ukraine's Russian-speaking eastern regions should be involved in drafting a constitution that should be put to the referendum.

Lavrov said it was not in Russia's interests for Ukraine to break up, but Moscow wanted all citizens of the country to be given equal treatment by Kiev.

He denied Ukrainian and U.S. allegations that Russia had undercover agents fomenting unrest in eastern Ukraine, and said he was seeking explanation of media reports that the director of the CIA, John Brennan, had visited Kiev.

The crisis in Ukraine has brought relations between Russia and the West to their worst since the end of the Cold War in 1991, and also risks unleashing a "gas war" which could disrupt energy supplies across Europe. Ukraine said on Saturday it would stop paying for Russian gas because the price was too high.

Russian stocks and the rouble fell sharply on Monday, reflecting fears of further Russian military intervention in Ukraine and more western sanctions against Moscow. {ID:nL6N0N61EC]

"CRIMINAL ORDER"

Use of force by Kiev's pro-Europe authorities could trigger a fresh confrontation from Russia. Russia's foreign ministry called the planned military operation a "criminal order" and said the West should bring its allies in Ukraine's government under control.

The United Nations Security Council held an emergency session on Sunday night, and the United States warned that it was likely to impose further sanctions on Russians close to the Kremlin if the escalation in eastern Ukraine continues.

British Foreign Secretary William Hague said the destabilisation of eastern Ukraine was clearly being instigated by Russia, adding: "I don't think denials of Russian involvement have a shred of credibility.

German Vice Chancellor Sigmar Gabriel warned of an escalation of the crisis in Ukraine, saying "Russia was clearly prepared to allow tanks to roll across European borders".

In Slaviansk as of 9 a.m. local time on Monday, a Russian flag still flew over police headquarters, one of two buildings taken over by the separatists in the town, while masked men continued to man barricades of sandbags in front of it.

Even as the deadline passed, a truck appeared bringing more tyres to heap on top of the barricades to reinforce them.