President Barack Obama is greeted by Lt. Gen. Curtis "Mike" Scaparrotti, left, and U.S. Ambassador to Afghanistan Ryan Crocker, second left, as he steps off Air Force One at Bagram Air Field in Afghanistan, Tuesday, May 1, 2012

US President Barack Obama has arrived in Afghanistan on a previously unannounced visit.

He is to sign an agreement with Afghan President Hamid Karzai charting future relations with the country.

The agreement will outline the US role in Afghanistan after 2014, when most Nato combat forces are due to pull out.

Mr Obama is also due to give a TV address to Americans back home. The visit comes on the first anniversary of Osama Bin Laden’s killing.

It was a year ago that US special forces carried out a raid on Bin Laden’s compound in Abbottabad, Pakistan, and killed the leader of the al-Qaeda network.

After Mr Obama’s arrival, Mr Karzai said a post-war agreement would seal an “equal partnership” between Afghanistan and the United States, the Associated Press (AP) news agency reports.

Mr Obama added the costs of war had been great and that the deal with Afghanistan paved the way for “a future of peace”, AP added.

The US is to designate Afghanistan as a major non-Nato ally, US officials are quoted as saying by Reuters news agency.

Mr Obama will not make specific decisions on further reductions of US forces in Afghanistan until the autumn of 2012, the officials added.

Mr Obama is due to make his TV address from Bagram air base at 23:30 GMT.

His trip is a first, symbolic step towards setting out a long-term relationship, says the BBC’s Paul Adams in Washington.

It is designed to reassure the people of Afghanistan that they are not about to be abandoned when Nato ends its operations there in 18 months.

It is also meant to send a signal to the Taliban that it cannot simply expect to take over again when the Americans leave, our correspondent adds.

This is President Obama’s third trip to Afghanistan since taking office.