(Recasts with Hagel comments, details on aid)
WASHINGTON, April 17 (Reuters) - The United States said on Thursday it would send Ukraine's armed forces medical supplies, sleeping mats and other non-lethal aid, seeking to signal support for Kiev while stopping far short of adding any U.S. weaponry to a deepening standoff with Russia.
Defense Secretary Chuck Hagel made the announcement at the Pentagon as Kiev grapples with a pro-Russia uprising that has seen fighters seize whole swathes of Ukraine while Moscow masses tens of thousands of troops on the frontier.
Hagel voiced deep concern over "Russia's ongoing destabilizing activities in eastern Ukraine."
"De-escalation has been our focus and Russia must take steps to make that happen," he said.
Russian President Vladimir Putin, who overturned decades of post-Cold War diplomacy last month by declaring Russia had a right to intervene in neighboring countries and annexing Ukraine's Crimea region, accused the authorities in Kiev on Thursday of plunging the country into an "abyss."
Hagel said he spoke on Thursday with his Ukrainian counterpart to inform him of additional non-lethal military assistance, including helmets and water purification units for Ukraine's armed forces.
The United States would also provide small power generators and hand fuel pumps for Ukraine's state border guard service, he said.
"The United States will continue to review additional support that we can provide to Ukraine," Hagel said.
Kiev has long asked for military support from the United States, which U.S. officials, speaking on condition of anonymity, have said included small arms, as well as non-lethal assistance, including military food rations.
But President Barack Obama's government has focused on providing financial support for Ukraine and pressuring Russia diplomatically, instead of trying to quickly overhaul Ukraine's heavily outnumbered and outgunned forces.
NATO has also made clear it will not intervene militarily in Ukraine, which is not a NATO member, despite Russia's annexation of Ukraine's Crimea region and a buildup of Russian forces on the Ukrainian border.
It is focusing instead on boosting temporarily its presence in eastern Europe in a drive to reassure allies, such as the ex-Soviet Republics in the Baltics, that NATO would protect them if they ever faced Russian aggression.
Speaking at a Pentagon news conference alongside Polish Defense Minister Tomasz Siemoniak, Hagel said the United States was looking at additional steps to reinforce NATO allies in central and eastern Europe.
He noted the United States would keep 12 F-16 fighter jets and 200 support personnel in Poland through the end of 2014.
His comments came a day after NATO announced it would send more ships, planes and troops to eastern Europe "within days."
(Reporting by Phil Stewart; Editing by Missy Ryan and Eric Beech)