Ukrainian acting president Oleksander Turchinov told eastern regions gripped by a pro-Russian uprising that they would be courting catastrophe if they voted "yes" in a separatist referendum today.

Turchinov, who deems the vote in the Russian-speaking Donetsk and Luhansk regions illegal, urged "round table" talks on greater autonomy. But, in reference to fighters who have seized police and government buildings, he said "terrorists" could not be included.

The vote, organised on a largely ad hoc basis with no clear control of ballot papers or voter lists, could have serious consequences for Ukraine and relations between Moscow and the West. It risks turning isolated clashes into civil war.

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Turchinov said on his website: "[Secession from Ukraine] ... would be a step into the abyss for these regions. Those who stand for self-rule do not understand that it would mean complete destruction of the economy, social programmes and life in general for the majority of the population in these regions."

In the port city of Mariupol, where between seven and 20 people were killed in fierce fighting on Friday, rebels blocked the streets yesterday with barricades.

Smoke was still coming from the partially burned-out administration building. There was no sign of Ukrainian forces.

The barricades were manned by a handful of pro-Russians, some with batons or clubs, wearing motorcycle helmets. No gunmen were visible. Video on the YouTube site showed an armoured car captured by rebels set on fire and ammunition exploding.

Across Sloviansk, the most heavily defended separatist city, streets were barricaded with tyres, furniture, cars and scrap iron.

In the city of Donetsk, rebels released several members of the Red Cross whom they had held for seven hours, one having been beaten, a Red Cross official said.

Western states prepared to step up pressure on Russia, who they accuse of engineering the crisis. Russia denies involvement but voices support for insurgents it says are defending themselves against fascist Ukrainian forces.

German Chancellor Angela Merkel and French President Francois Hollande said that if the May 25 national polls failed to go ahead because of the rebellion, this would further unsettle the country. In that case, they would be "ready to take further sanctions against Russia".

The European Union has so far imposed asset freezes and visa bans on 48 Russians and Ukrainians over Moscow's annexation of Crimea. EU diplomats say new sanctions will for the first time target companies.

The national polls are seen in Kiev as a way of establishing a fully legitimate government, following pro-Kremlin president Viktor Yanukovich's flight to Russia in February under pressure from pro-western demonstrations.

In the largely rebel-controlled regions of Donetsk and Luhansk, which have declared a breakaway "People's Republic of Donetsk", preparations went ahead for today's self-rule referendum, though there was widespread uncertainty about the meaning of the question on the ballot paper: "Do you support the act of self-rule of the People's Republic of Donetsk?"

Some people interpret it as a vote for more local powers, some for broad autonomy within Ukraine, some for independence. Others still see it as a step towards incorporation into Russia.