America's new Secretary of State John Kerry will hold talks with the Prime Minister and Foreign Secretary William Hague on the first stop in an 11-day world tour.
The conflict in Syria, nuclear stand-off with Iran and Middle East peace process are expected to be high on the agenda.
But the Falklands is also likely to come up - with Washington under pressure to recognise the result of a referendum next month on whether the islanders want to remain British.
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The vessel went missing last week with 44 sailors on board.
Joined by 18 Labour MPs, McDonnell voted with 283 Conservatives, eight DUP and two independents to defeat the Taxation Bill amendment.
Gerry Adams has told UTV the “time is right” for him to step aside as president of Sinn Féin, after announcing that he would do so in 2018.