Ukraine's acting president Oleksandr Turchynov is promising amnesty for pro-Russian demonstrators if they give up their weapons and abandon government buildings under siege in two eastern Ukrainian cities.

Mr. Turchynov said Thursday he is willing to make his promise official in a presidential proclamation.

There has been no reply so far from the demonstrators, who took over official buildings in the cities of Luhansk and Donetsk. They are demanding referendums on whether to split with Ukraine and join Russia -- the same vote that was held in Crimea last month.

On Wednesday, Ukraine's Interior Minister Arsen Avakov said the occupation of the buildings would be ended Friday, either by negotiations or by force.

Meanwhile, President Vladimir Putin has sent a letter to European leaders hinting that if the issue of Ukraine's natural gas debt to Russia is not resolved, Russian natural gas deliveries to Europe, which cross Ukraine, could be disrupted.

Russia's state news agency RIA Novosti quotes a presidential spokesman, Dmitry Peskov, as saying the letter expressed "extreme concern" about Ukraine's gas debt -- "and, accordingly, deliveries of Russian gas."

Peskov said the letter called for "urgent measures" but he refused to provide any specifics.

U.S. diplomat Victoria Nuland told a U.S. Congressional committee Wednesday that the building takeovers in eastern Urkaine were "very carefully orchestrated, well-planned, well-targeted" moves. She said there is "overwhelming evidence" of Russian involvement, and warned of consequences if the actions go unchecked.

Nuland told the lawmakers that U.S. diplomats do not hold out high expectations for talks next week involving top diplomats from Ukraine, Russia, the United States, and the European Union.