Pro-Russian groups are dug-in around Donetsk with barbed wire and molotov cocktails

Pro-Russian protesters were engaged in a standoff with Ukrainian forces in Donetsk on Tuesday, after occupying an administrative building over the weekend and declaring the eastern Ukrainian city a separatist republic.

Their main demand was a referendum on joining Russia — as the Ukrainian region of Crimea did in March.

Ukraine's security agency accused pro-Russian activists in the eastern city of Luhansk of acting like terrorists, saying its investigators had found explosives in the main state security building that the protesters had stormed on Sunday.

Interior Minister Arsen Avakov blamed Russian President Vladimir Putin for the unrest. The United States also accused Russia of attempting to destabilize Ukraine, a claim that Russian Foreign Minister Sergei Lavrov dismissed.

While the United States and NATO warned Russia of consequences if it intervened further, Ukraine had more pressing concerns about Russian producer Gazprom cutting off gas supplies as it did in the winters of 2005-6 and 2008-9.