Germany and France have threatened Russia with further sanctions if the 25 May election in Ukraine does not go ahead as planned.

The German chancellor, Angela Merkel, urged the Russian president, Vladimir Putin, to do more to calm the situation, warning that a separatist referendum being held on Sunday would be "illegal".

In a joint statement with the French president François Hollande, Merkel said Putin must "send more signals of de-escalation so that [presidential] elections can take place.

"We are ready to take further sanctions against Russia," the two leaders warned. "There have been initial signs, but this must be strengthened so that the message gets through to eastern and southern parts of Ukraine that everyone wants fair and general presidential elections."

Merkel added: "If that doesn't happen, it would contribute to a further destabilisation of Ukraine."

Hollande, who met Merkel in her Stralsund constituency in north-eastern Germany, called for a "national dialogue" between Kiev and representatives of Ukrainian regions. He said: "We have relations with Vladimir Putin; we are using them so that he can take into consideration what is at stake over these coming weeks in Ukraine."

Oleksandr Turchynov, Ukraine's acting president, warned eastern regions that voting for self-rule could cause a catastrophe. Photograph: ITAR-TASS/Barcroft Media

The threat came as Ukraine's acting president, Oleksander Turchynov, warned eastern regions gripped by the pro-Russian uprising that voting yes in a separate sovereignty referendum on Sunday could cause a catastrophe. He urged people to accept round table talks but said "terrorists" could not be included.

Turchinov said on his website: "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."

There were no reports of fighting on Saturday, but the atmosphere in major cities across the region was tense. Rebels have blocked the streets with barricades in the port city of Mariupol, where between seven and 20 people were killed in fierce fighting on Friday, and smoke was still coming from the partially burnt-out administration building. There was no sign of Ukrainian forces.

A Red Cross official said several of their members had been released in the city of Donetsk, where they had been held for seven hours, one having been beaten.

Sunday's referendum has been organised on a largely ad hoc basis, with no clear control of the authenticity of ballot papers or voter lists. Merkel described the vote as illegal, while said it "carried no weight".

Turchinov said a yes vote would be "a step into the abyss" for the eastern regions.

The European Union has already handed out targeted sanctions on Russian officials but has refrained from imposing all-out economic sanctions.