KIEV — US Secretary of State Mike Pompeo said Friday Washington's support for Ukraine was unwavering during a delicate visit to the country at the heart of the US impeachment process.

After talks with Pompeo, Ukrainian President Volodymyr Zelensky said he appreciated US support, but that Washington should be more involved in resolving his country's conflict with Moscow-backed separatists.

Ukraine is walking a careful line in its relations with the United States — keen to maintain US backing in the war but wary of being further entangled in US domestic politics.

Pompeo is the most senior US official to visit Ukraine since the start of the impeachment process against US President Donald Trump, who is accused of withholding military aid to push Kiev into investigating his Democratic rival Joe Biden.

Pompeo said there could be no doubts about Washington's support for Ukraine, calling it "a bulwark between freedom and authoritarianism in eastern Europe".

"The United States sees that the Ukrainian struggle for freedom, democracy and prosperity is a valiant one. Our commitment to support it will not waiver," he said at a press conference with Zelensky.

Zelensky, who allegedly faced the pressure from Trump in a July 25 phone call, called on Washington to "be more actively involved in the peace process in eastern Ukraine" and to appoint a special representative on the conflict.

Washington has been an important backer of Ukraine in its conflict with the separatists in the country's east, providing both military aid and diplomatic support.

That support is at the center of the impeachment process, with Trump accused of withholding $400 million in assistance to demand dirt on Biden, whose son was on the board of a Ukrainian company.

A Senate trial of Trump is underway in Washington, but the Republican majority in the US upper house all but guarantees an acquittal. A vote to acquit could come as soon as Friday.

Pompeo is himself under pressure over Ukraine. He was among senior officials who listened in on the phone call with Zelensky and has been accused of not doing enough to defend Marie Yovanovitch, the former US envoy to Kiev who Trump removed last year.

Pompeo Friday also met with Ukraine's Foreign Minister Vadym Prystaiko and the head of the newly independent Ukrainian Orthodox Church, Metropolitan Yepifaniy.

He visited Kiev's golden-domed Saint Michael's Cathedral — torn down by the Soviet Union but rebuilt after Ukraine's independence — and a memorial wall with photographs of soldiers killed in the conflict in the country's east that broke out in 2014.

Washington's support for Kiev has included not only military assistance but diplomatic backing, including the imposition of US sanctions against Russia following its 2014 annexation of Crimea.

Zelensky, a comedian with no previous political experience who won a landslide election victory last year, has made ending the conflict a key plank of his presidency.

The fighting with separatists in two mainly Russian-speaking breakaway regions has left more than 13,000 dead and crippled Ukraine's economy.

Efforts to resolve the conflict were largely frozen under Zelensky's predecessor Petro Poroshenko, but the new president has taken several key steps since coming to power.

Forces from both sides have pulled back from several areas of the frontline, two exchanges have seen dozens of prisoners swapped and in December Zelensky met for the first time with Russian President Vladimir Putin at a summit in Paris.

Russia is accused of backing the separatists and seeking to undermine Ukraine's efforts to develop closer ties with the West.

After Kiev, Pompeo was due to visit three more countries of the former Soviet Union, seeking to extend US influence in Russia's traditional sphere.

He was to meet with senior officials including President Alexander Lukashenko in the Belarusian capital Minsk on Saturday, then travel on to Kazakhstan and Uzbekistan in Central Asia.

Asked about Pompeo's tour on Friday, Kremlin spokesman Dmitry Peskov showed little concern.

"It is natural for the United States to have relations with these countries," he told reporters. "And it is the sovereign right of every country to develop international relations." — AFP