Barack Obama pledges 'responsible' end to 13-year war during surprise visit to troops in Afghanistan

United States president Barack Obama said America's 13-year war in Afghanistan was on track to come to a "responsible end" during a surprise visit to troops in the country.

The trip, timed to coincide with Memorial Day, comes as the president prepares to make a final decision about how many troops will be left in Afghanistan beyond the end of the year.

During the visit, which included a military briefing and a visit to wounded soldiers in hospital, Mr Obama said a decision about troop numbers would be made "fairly shortly".

"Once Afghanistan has sworn in its new president, I'm hopeful we'll sign a bilateral security agreement," Mr Obama said.

"Assuming it is signed, we can plan for a limited military presence in Afghanistan beyond 2014.

"And by the end of this year, the transition will be complete and Afghans will take full responsibility for their security and our combat mission will be over. America's war in Afghanistan will come to a responsible end."

Mr Obama also planned to meet top US military commanders and civilian leaders in Afghanistan during his brief visit.

However, Afghan president Hamid Karzai was offered a meeting with Mr Obama during the brief visit but declined, a US official said.

The official said that Washington was not surprised the meeting did not work out at short notice, after Mr Obama arrived at the base on a surprise visit after night fell.

"As we said, we weren't planning for a bilateral meeting with president Karzai or a trip to the palace, as this trip is focused on thanking our troops," the official said.

"We did offer him the opportunity to come to Bagram, but we're not surprised that it didn't work on short notice.

"The president will likely be speaking by phone with president Karzai in the days to come, and also looks forward to working with Afghanistan's next president after the election is complete."

Mr Obama called Mr Karzai from Air Force One after departing Afghanistan to discuss progress made by Afghan security forces and US troop numbers after 2014.

Elections to resolve security agreement deadlock

Mr Obama and Mr Karzai have a testy relationship and Washington has been deeply frustrated by the outgoing Afghan leader's refusal to sign a bilateral security agreement that would allow it to lock in a post-2014 US troop training mission in the country.

However, US officials said they believed that both candidates in Afghanistan's run-off election, Ashraf Ghani and Abdullah Abdullah, would sign a bilateral security agreement that would allow US troops to stay.

But Washington has also warned that without the agreement - which Mr Karzai has refused to sign - it may be forced to go to a "zero option" of no troops.

Mr Obama said the recent first round of the Afghan election had been a success, and praised the work of building up Afghan forces.

"I will be honest with you, it has gone better than I might have expected a year ago," he said.

The president has brought American country music star Brad Paisley with him to provide entertainment for the troops.

The surprise trip, pulled off under extraordinary security precautions, comes ahead of Memorial Day, when the United States marks the sacrifices of its war dead and honours veterans.

ABC/wires