Prime Minister David Cameron and President Obama have hailed the "special relationship" between the UK and America as essential for the world.
External site: US State Visit photos on Flickr
They held talks in Number 10 as President Obama visited the UK for an official State Visit at the invitation of Her Majesty The Queen.
In a joint press conference following their meeting, Mr Cameron praised the working partnership between London and Washington.
The Prime Minister said:
…in millions of interactions every day - including our massive business relationship - our people forge friendships together. That’s what makes this relationship special.
But what makes it essential is that it’s not just about history or sentiment.
It is a living, working partnership. It is essential to our security. It is essential for our prosperity.
Discussing the NATO operation in Libya, the two leaders also called on Muammar Qadhafi to step down from power. They said air strikes would continue against targets in Libya until he left the country to its people.
We will continue those operations until Qadhafi’s attacks on civilians cease. Time is working against Qadhafi and he must step down from power and leave Libya to the Libyan people.
They also both reaffirmed their commitment to ensuring the Arab Spring continues and called on other leaders of Arab and North African countries to pave the way for political reforms.
We are seeing some extraordinary things: protesters braving bullets, bloggers toppling dictators, people taking to the streets and making their own history.
If global politics is about spreading peace and prosperity, then this is a once-in-a-generation moment to grab hold of. It is not a time for us to shrink back and think about our own interests.
On Wednesday, the PM and President Obama announced six specific areas where the United Kingdom and the United States will strengthen their co-operation in the coming months.
They span security and support to Armed Forces personnel; commitments to collaboration in science, higher education, volunteerism and international development; and the development of cyberspace.