MOSCOW: Russia’s President Vladimir Putin has sent a letter to 18 European leaders urging them to offer a quick financial assistance to Ukraine to prevent a shutdown of Russian natural gas to much of Europe.

In a letter released Thursday by the Kremlin, Putin warned that Ukraine’s mounting gas debt is forcing Moscow to start demanding advance payments for Russian gas supplies. He warned that if Ukraine fails to make such payments, Russia’s state-controlled gas giant Gazprom will “completely or partially cease gas deliveries.”

Putin told the leaders that a possible shutdown of Russian gas supplies will increase the risk of Ukraine siphoning off gas that intended for Europe and will make it difficult to accumulate sufficient reserves for next winter.

Meanwhile, Ukraine’s acting president promised Thursday that pro-Russian activists occupying government buildings in the eastern cities of Donetsk and Luhansk will not be prosecuted if they lay down their arms.

The comments by Oleksandr Turchynov came as NATO’s chief visited Prague to assure the Czechs of support against any moves by Russia to expand its orbit of influence in the region.

Turchynov, speaking to the Ukrainian Parliament in Kiev, commended the activists in Luhansk, who have begun negotiating with national authorities in Kiev and encouraged Donetsk to follow their example.

Hundreds of armed pro-Russian protesters have occupied a regional government administration building in Donetsk and a branch of the Ukrainian Security Service in Luhansk since Sunday. They have demanded a referendum on broader autonomy or even secession from the new Kiev authorities, who took power after Kremlin-friendly President Viktor Yanukovych fled the country in February following months of street protests.

Tensions were still high Thursday in Donetsk, with about 1,000 protesters standing outside the occupied building chanting “Russia! Russia!” Activists were reinforcing lines of barricades, piling up rubber tires or building brick walls out of cobblestones.