SOUTHEASTERN POLAND, April 25 (Reuters) - Washington's top diplomat and defense secretary both visited Kyiv on Sunday, and used the first official U.S. visit to Ukraine since Russia invaded two months ago to announce a gradual return of U.S. diplomats to the country and the nomination of a new ambassador, officials said.

Secretary of State Antony Blinken and Secretary of Defense Lloyd Austin traveled to Poland on Saturday then overland into Ukraine on Sunday, where they met President Volodomyr Zelenskiy and other top Ukrainian officials, a senior State Department official said, declining to discuss in detail their travel or security arrangements.

The visit was designed to show Western support for Ukraine and the cabinet secretaries also pledged new assistance worth $713 million for Zelenskiy's government and countries in the region, where Russia's invasion has raised fears of further aggression by Moscow.

It also highlighted the shift in the conflict since Ukrainian forces, armed with a massive influx of weapons from the West, successfully repelled a Russian assault on Kyiv.

Russian forces have regrouped to try to capture more territory in the southeastern Donbas region, letting foreign leaders visit the capital and some Western nations resume their diplomatic presence in recent weeks, but Washington has been cautious about a return amid sporadic Russian missile attacks in the west.

Officials declined requests from the media to accompany the secretaries into Ukraine, citing security concerns. The officials briefed reporters in Poland on condition the trip not be reported until the delegation was safely out of Ukraine.

Austin will travel on to Germany, where he will host counterparts from more than 20 nations and NATO Secretary General Jens Stoltenberg at the U.S. air base at Ramstein to discuss Ukraine's defense needs, a Pentagon official said.

Zelenskiy announced the trip himself during a news conference on Saturday, which U.S. officials said was a contingency they had planned for and did not derail the visit.

Blinken and Austin were expected to have met for about 90 minutes with Zelenskiy, Ukraine's Foreign Minister Dmytro Kuleba, Defence Minister Oleksii Reznikov and other officials.

U.S. diplomats departed the Kyiv embassy nearly two weeks before the Feb. 24 invasion, moving some functions to the western city of Lviv before eventually relocating to Poland.

Diplomats will initially resume "day trips" across the border to Lviv in the coming week and officials are accelerating planning to return to the Kyiv mission, the State Department official said.