US military experts on Monday began training Ukrainian soldiers and special operations forces in the war-torn country, the Pentagon said.

US troops had already deployed in small numbers to Ukraine to train National Guard forces, but under a plan first announced in July they are now helping regular military units.

The US troops "will be training five battalions of active-duty troops and one battalion of special operations forces personnel," spokesman Captain Jeff Davis said.

More than 8,000 people have been killed since pro-Russian insurgents took up arms against Ukraine's pro-Western government last year following the ouster of a Kremlin-backed president in the capital Kiev.

Washington has made repeated pledges of support for Ukraine, and has committed more than $265 million in training and equipment since 2014, Davis said.

"This is part of our ongoing efforts to contribute to Ukraine's long-term military reform and professionalism and to help them improve Ukraine's internal defense capabilities and training capacity," he added.

The United States and its NATO allies have backed Ukraine diplomatically and have denounced what they see as Russia's intervention to support the rebels and prolong the destabilizing conflict.

But they have been cautious not to be drawn directly into military confrontation with Moscow, offering only limited training support for Kiev's units rather than arms shipments.

The United States has sent non-lethal military aid including Humvees, counter-mortar radars, night vision goggles, body armor and medical equipment to Ukraine.