KABUL, May 25  -- U.S. President Barack Obama paid a surprise visit to Afghanistan on Sunday, an Afghan official told Xinhua.

The U.S. president arrived in the Bagram air base in north of Kabul, the main U.S. base in Afghanistan, late on Sunday, the source said on condition of anonymity.

The aim of his trip is reported to meet American soldiers on a memorial day weekend and he will also visit the injured soldiers who got wounded in Afghanistan war and conflicts.

Obama is not planning to meet President Hamid Karzai and the two leading presidential candidates of upcoming presidential runoff, local media reported.

The source said the U.S. leader would stay for a short five- hour trip and that he will not make announcement on the number of Americans troops in Afghanistan after 2014. However the Associated Press quoted Obama as saying "We'll probably be announcing some decisions fairly shortly," after a briefing by Marine Gen. Joseph F. Dunford, the top American commander in Afghanistan.

More than 51,000 NATO-led troops, including 33,500 American forces, are currently stationed in Afghanistan. The majority of the foreign troops are set to leave the country by the year end.

Kabul and Washington failed to sign the controversial Bilateral Security Agreement (BSA) which would govern the presence of U.S. troops in Afghanistan after 2014.

The Afghan grand assembly attended by 2,500 tribal elders and notables from across the country in November last year endorsed the BSA. However, Karzai said he would sign the pact only if the U. S. agreed to support meaningful peace talks with the Taliban and stop searching civilian houses.

Washington urged outgoing Karzai to sign the BSA before the end of 2013, warning that failure to sign the pact would mean that U.S. forces completely leave the country and Afghanistan risks renewed civil war.

The two presidential runoff candidates, Abdullah Abdullah and Ashraf Ghani Ahmadzai, have said they would sign the BSA if they win the election.