Russia's Vladimir Putin has decided to invade Ukraine within days, U.S. President Joe Biden said on Friday, February 18 after separatists backed by Moscow told civilians to leave breakaway regions on buses, a move the West fears is part of a pretext for an attack, Reuters reports.

Warning sirens blared in Donetsk and Luhansk on Friday after rebel leaders there announced the evacuation of hundreds of thousands of people to Russia.

"We have reason to believe the Russian forces are planning to and intend to attack Ukraine in the coming week, in the coming days," Biden told reporters at the White House, adding that Kyiv would be a target.

"As of this moment, I am convinced that he has made the decision."

Late on Friday, Ukraine's military intelligence said Russian special forces had planted explosives at social infrastructure facilities in Donetsk, and it urged residents to stay at home. The Russian Federal Security Service did not immediately reply to a request for comment.

Citing correspondents on the ground, Russian news agencies later reported that two explosions hit Luhansk, one of the main cities in Ukraine's breakaway People's Republic of Luhansk, and a section of a gas pipeline in the area caught fire.

Earlier, separatist leaders in Donetsk and Luhansk issued video statements in which they announced the evacuations and accused Ukraine of preparing to attack both regions soon - an accusation Kyiv said was false.