The Russian military is planning to increase the use of small squads on motorcycles and quadbikes on the frontlines in Ukraine as it plans fresh offensives, according to the Ukrainian military and analysts of the conflict.

Russia launched a sweeping drone assault and airstrikes across Ukraine early Sunday, killing at least four people, officials said, after U.S. President Donald Trump cast doubt over Russian President Vladimir Putin’s willingness to end the war.

U.S. Secretary of State Marco Rubio said the U.S. would need to “make a determination about whether this is an endeavor that we want to continue to be involved in.”