US Secretary of State Mike Pompeo visits Kiev on Friday on his first trip to Ukraine, the country at the heart of President Donald Trump's impeachment trial.

The visit, which was scheduled for early January but then delayed because of Middle East tensions, is expected to highlight US support for Ukraine in its conflict with Russia-backed separatists, but also in its efforts to root out corruption.

"(I will) talk about the important work that the United States and Ukraine will continue to do together to fight corruption inside of that country," Pompeo told reporters on Wednesday.

"And to ensure that America provides the support that the Ukrainian people need to ensure that they have a free and independent nation," he said aboard his jet, travelling to Kiev with a 24-hour stopover in London.

Pompeo will meet President Volodymyr Zelensky, who is at the centre of Trump's historic impeachment trial for abuse of power and obstruction of Congress. 

The Democratic prosecution team at the Senate trial has alleged Trump flagrantly tried to force Kiev to help him tarnish his possible election challenger, former vice president Joe Biden.

Key to the impeachment scandal is a July 25 telephone call in which Trump pushed Zelensky to announce an investigation of Biden.

Trump is accused of withholding nearly $400 million in military aid for Ukraine in the war in the east of the country, and refusing Zelensky a White House meeting unless he opened the probe.

American financial and military aid is crucial for Kiev, and that is a reason why Ukraine was struggling not to be involved in the US campaign, trying to maintain support from both American major parties.

- 'Very cautious' -
"Pompeo will be very cautious, considering how sensitive this topic is," Oleksiy Melnyk, a foreign policy analyst at the Razumkov Centre in Kiev, said regarding Trump's alleged pressure on Ukraine's leader.

Pompeo has declined to comment whether he intends to raise the Biden subject during the visit.

"I do not want to talk about particular individuals. It is not worth it," Pompeo said, adding that he will discuss Zelensky's "committment" to fight corruption.

The secretary of state came under increased scrutiny last year when it emerged that he had been one of the senior administration officials listening in on Trump's July 25 phone call with Zelensky.

Trump last year removed the ambassador to Ukraine, Marie Yovanovitch, and denounced her in the call with Zelensky.

Former career diplomats accused Pompeo, a stalwart ally of Trump, of not robustly defending Yovanovitch either to the White House or in public.

Pompeo's stance on Yovanovitch became a subject of his feud with journalists this week as a correspondent for National Public Radio in the United States has been excluded from the Secretary of State's plane, after a tense exchange between him and another NPR journalist.

During his visit, Pompeo will also meet Ukraine's foreign minister and defence minister, the leader of the new independent Ukrainian Orthodox Church Metropolitan Yepifaniy and brief reporters after the talks with Zelensky.

Early Saturday, Pompeo will continue his trip in the region, heading to Belarus, and then to Central Asia's Kazakhstan and Uzbekistan.