US Secretary of State Mike Pompeo. Photo: IC

US Secretary of State Mike Pompeo underscored Washington's support for Ukraine on a visit to Kiev on Friday, as both countries sought to smooth over relations buffeted by the impeachment trial of President Donald Trump.

Ukraine was thrust center stage in a domestic political battle in Washington last year as Trump faced down allegations that he pressured Ukrainian President Volodymyr Zelensky to investigate the son of his Democratic rival Joe Biden.

Pompeo has also walked back from comments he was accused of making to a National Public Radio reporter ahead of his trip that appeared to play down Ukraine's importance to Americans.

Pompeo is the most senior US official to travel to Ukraine since the impeachment process began, and his visit comes just as Trump's impeachment trial appeared set to wrap up as early as this weekend.

Ukraine counts on Washington for diplomatic support and military aid to buy Javelin anti-tank missiles and other hardware as it battles Russian-backed fighters.

Trump temporarily froze nearly $400 million in US security assistance shortly before speaking to Zelensky in a July phone call, prompting accusations from Democrats he had misused US foreign policy for personal gain.

Trump's camp has called on Ukraine to investigate Biden and his son Hunter Biden, who was on the board of a Ukrainian energy company called Burisma.

Democrats had hoped to hear from former National ­Security Adviser John Bolton as a witness in Trump's impeachment trial.

A report, which Bolton has not denied, said Bolton in an upcoming book has ­written that Trump told him he wanted to freeze military aid to Ukraine until it investigated Biden and his son Hunter.

Trump has denied any wrongdoing and called the impeachment process a "witch hunt."