Mrs Clinton is on a five-day, six-city diplomatic tour of Europe
US Secretary of State Hillary Clinton has arrived in the UK on the second leg of a five-day tour of Europe.
The Afghan war and concerns over Iran's nuclear programme are expected to be high on the agenda when she meets Prime Minister Gordon Brown on Sunday.
Mrs Clinton arrived from Zurich in Switzerland, where she attended the signing of a historic pact between Turkey and Armenia.
The two countries normalised relations after a century of hostility.
US Assistant Secretary of State for European and Eurasian Affairs Philip Gordon said that while in Britain, Mrs Clinton would "talk with senior UK officials on a wide range of bilateral and transatlantic issues, including Iran, Afghanistan and Pakistan".
Britain has the second largest troop deployment in Afghanistan after the US.
President Barack Obama is currently reviewing the strategy for Afghanistan and spoke with Prime Minister Gordon Brown on Thursday to discuss the issues.
On Monday, the US secretary of state will address the Northern Ireland assembly in Belfast.
Visiting Russia for the first time as chief US diplomat, Mrs Clinton will hold talks on Iran's disputed nuclear programme with Russian President Dmitry Medvedev in Moscow on Tuesday.
She is expected to press the Russian leader for a strong commitment for tough new sanctions on Tehran.
Officials say she will also raise other issues, including arms control, missile defence, and cooperation to convince North Korea to abandon nuclear weapons.
Mrs Clinton returns to Washington late on Wednesday, after visiting the Russian city of Kazan.