US Secretary of State Mike Pompeo heads to Britain next week ahead of its exit from the European Union and will then travel to Ukraine, the country at the heart of President Donald Trump's impeachment trial.

Pompeo arrives in London on Wednesday, meeting with Prime Minister Boris Johnson and Foreign Secretary Dominic Raab to "discuss ways to broaden and deepen trade ties" after Brexit at the end of the month, the State Department said.

Johnson is cultivating Britain's relations with Washington in a bid to offset the potential damage of withdrawal from the EU.

But there are several points of friction.

Trump, a longtime advocate of Britain's leaving the European Union, has praised Johnson and promised a lucrative new trade deal, although both sides admit hammering out the details will be hard.

Pompeo's visit coincides with an expected British decision on whether to allow Chinese telecom giant Huawei to develop its next-generation 5G phone network in the country, something the US has strongly discouraged as a security threat.

Huawei denies such accusations.

The British government on Friday rebuked the US for refusing to extradite a diplomat's wife charged over a car crash that killed a British teenager.

Raab called it a "denial of justice."

The US had rejected the extradition request, and Pompeo's department said that Anne Sacoolas, as a diplomatic spouse, enjoyed immunity from prosecution.

Harry Dunn, 19, died in August when his motorcycle collided with a car driven by Sacoolas on the wrong side of the road in Britain.

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

The visit had been expected in early January but was delayed because of Middle East tensions.

The Democratic prosecution team at the Senate trial has alleged Trump flagrantly tried to force Kiev to help him tarnish his possible 2020 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's war against Russian-backed separatists, and refusing Zelensky a White House meeting unless he opened the probe.

In Kiev, Pompeo will attend a wreath-laying ceremony at St Michael's cathedral to honor Ukrainians killed in the war against Russian-back separatists in the eastern region of Donbas, the State Department said.

It added the trip will "highlight US support for Ukraine's sovereignty and territorial integrity."

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.

After his Ukraine trip, Pompeo will head to Belarus, Kazakhstan, and Uzbekistan, where he will meet the leaders of each country.