SLAVYANSK, Ukraine (AFP) - Russia rejected a new peace initiative for Ukraine on Tuesday as fears of open war mounted in the ex-Soviet republic, whose troops are waging a deadly offensive against pro-Moscow rebels.

French President Francois Hollande and German Foreign Minister Frank-Walter Steinmeier added their voices to the chorus warning that skirmishing in the east of the country could trigger a civil war.

Underlining the risk, Kiev announced that the death toll from an assault on a rebel-held flashpoint town had climbed to more than 34.

The West, which is threatening to broaden sanctions on Russia over the worsening crisis, sees a May 25 presidential poll in Ukraine as crucial to hauling the country back from the brink.

On Tuesday, the Swiss presidency - which chairs the Organisation for Security and Cooperation in Europe - urged a suspension of hostilities for that election to take place.

Ukraine's foreign minister Andriy Deshchytsia urged 30 of his counterparts assembled in Vienna to help "eliminate the external threats and provocations supported by Russia."

But Russian Foreign Minister Sergei Lavrov, also at the Vienna meeting of the Council of Europe, said holding the vote during the current violence would be "unusual".

President Vladimir Putin's spokesman has been more blunt, calling the idea "absurd".

Mr Lavrov also dismissed a European push spearheaded by Germany to hold fresh peace talks on Ukraine after the collapse of an April 17 agreement that Moscow has already declared dead.

Any talks excluding the pro-Russian rebels active in Ukraine's east and southeast "would hardly add anything," he said.

The diplomatic impasse came as bloodshed soared in Ukraine.

Nearly 90 people have died in less than a week: half around the eastern town of Slavyansk, held by rebels since early April; and half in the southern port city of Odessa, where clashes culminated in a deadly inferno last Friday.

Most of those killed have been pro-Russian fighters and activists.

Mr Putin has so far not ordered a threatened operation to "protect" Ukraine's Russian-speaking population, but he has kept an estimated 40,000 troops on the border for the past two months.

The US general commanding Nato's military operations, Philip Breedlove, said Russian special forces the West alleges are deployed covertly "may be able to accomplish his (Putin's) objectives in eastern Ukraine" without the need to invade.

Kiev and its Western backers see Moscow's main aims as making sure Ukraine's east holds a planned "referendum" for independence on Sunday, and sabotaging all possibility of the nationwide presidential election two weeks later.

US Secretary of State John Kerry slammed the efforts to hold a referendum in the eastern cities of Donetsk and Luhansk.

"We flatly reject this illegal effort to further divide Ukraine," he said, after meeting with EU foreign policy chief Catherine Ashton.

"We are not going to sit idly by while Russian elements fan the flames of instability instead of fulfilling the commitments that we made."

As the two votes near, Ukraine's authorities are stepping up their offensive to crush rebels holed up in Slavyansk, a town of more than 110,000 people that is the epicentre of the insurgency.

Four members of the Ukrainian security forces were killed and 20 wounded there in fierce fighting on Monday, Interior Minister Arsen Avakov said on his official Facebook page.

"According to our estimates over 30 terrorists were killed and dozens were wounded," he added.

In a sign of how well-armed and trained the pro-Russian fighters are, on Monday they shot down a helicopter gunship near the town - their third since the assault on the town began last week.

An AFP reporter inside Slavyansk said the rebels were reinforcing defences around a security service building they occupy. He said basic foodstuffs and other items were running increasingly short.

Russia says the insurgency in Ukraine is a spontaneous rejection of the Kiev government that came to power in February, after street protests forced out the pro-Kremlin president.

Russian state media, seen in east Ukraine, constantly refer to the new administration as being run by "fascists" who embrace a Nazi-style ideology.

Their rhetoric will likely become even more shrill on Friday, when Ukraine - like Russia - observes a holiday commemorating the Soviet victory over wartime Germany.

Mr Putin, according to Russian reports, could make a triumphant visit to annexed Crimea on that day, after overseeing a parade of Russian military might on the Red Square in Moscow.

German Chancellor Angela Merkel, Europe's pointwoman in dealing with Putin over the crisis, said such a trip by the Russian leader would be a "pity".

Relations between the West and Russia are already plumbing depths not seen since the Cold War.

Further stoking tensions are threats by Russia to cut gas to Ukraine over unpaid bills, which could wreak havoc in Europe.

Energy ministers from the G-7 group of leading economies meeting in Rome said they would help Ukraine strengthen its energy security.

"Energy should not be used as a means of political coercion nor as a threat to security," said the ministers, from Britain, Canada, France, Germany, Italy, Japan and the United States.

The United States and the European Union are already applying sanctions pressure on targeted individuals and firms in Russia.

Washington says they will be expanded to punish whole sectors of Russia's slowing economy if Ukraine's May 25 presidential election is disrupted or if Russian troops invade.