David Cameron and Barack Obama have described the "unique and essential" relationship between the US and UK ahead of a visit by the prime minister to Washington.
In a joint article for the Washington Post, the two leaders said "the world counts on our alliance".
During the three-day trip they will discuss what they call "the next phase of the transition" in Afghanistan.
Tensions over Iran and the violence in Syria will also be high on the agenda.
Cameron made his first official visit to the US as prime minister in July 2010.
The latest meeting comes ahead of NATO and G8 summits.