Russia's foreign minister has ruled out holding fresh talks in Geneva to defuse the Ukraine crisis, unless pro-Russian opposition groups are involved.

Russia's foreign minister has ruled out holding fresh talks in Geneva to defuse the Ukraine crisis, unless pro-Russian opposition groups are involved.

Sergei Lavrov added there was no point as an April accord between the US, EU and Russia had not been implemented.

He spoke after a Council of Europe meeting which was expected to support Ukraine's plans for a 25 May election.

But Mr Lavrov called an election "unusual" at a time when the army was being used against the population.

Ukrainian acting Foreign Minister Andriy Deshchytsia said Ukraine was ready to back a new round of talks in Geneva as long as Moscow supported presidential elections.

"If Russia is ready to commit itself to support these elections and to eliminate this threat and eliminate its support for the extremist elements in Ukraine, we are ready to have such a round of meetings," he said at a news conference.

In recent days, security forces have launched a crackdown on pro-Russian separatists in the east of the country, triggering clashes outside the town of Sloviansk.

Interior Minister Arsen Avakov said on Tuesday that four soldiers and an estimated 30 separatists had been killed in the "anti-terrorism operation".

He said that up to 800 well-trained militants armed with large-calibre weapons and mortars were hiding among civilians in the town, where government buildings have been seized and checkpoints set up.

A Russia Today reporter in Sloviansk said residents were stockpiling food and there were barricades on almost every road.

Meanwhile, many flights in and out of Donetsk were suspended. The Ukrainian aviation authorities gave no reason for the move.

New checkpoints were earlier set up around Kiev. The interior ministry said it wanted to prevent the movement of weapons and explosives.

Russia's goals in this crisis have not changed. Moscow insists that it is simply acting to protect fellow Russian speakers. But Western leaders believe that it is intent upon wrecking any chance of holding truly national presidential elections in Ukraine later this month. Its longer-term aim is to undermine and weaken the government in Kiev.

However, on the ground, the balance of advantage may be subtly changing. While many buildings and road blocks remain in separatist hands, the Kiev government's ability to mount a reasonably effective security operation has raised the stakes for Moscow.

Russia hoped that by massing troops on Ukraine's frontier while seeking to infiltrate and undermine Kiev's authority from within, it could achieve a looser, more decentralised Ukraine. For a while, this seemed to be working. Kiev's authority was cowed and the willingness of its troops to fight was questionable.

That seems to have changed; Kiev is in effect calling Moscow's military bluff, bringing closer the moment when Russian President Vladimir Putin must decide whether or not to use overt military force.