United States President Barack Obama made a surprise visit to U.S. forces stationed in Afghanistan on Sunday and thanked the troops for their service to their country.

During the Memorial Day weekend visit, Obama noted the shifting role the troops played as their combat operations come to a close by year-end.

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The president addressed the troops at Bagram Air Field, and said that this is the last tour in Afghanistan for many of them.

Obama said that there will be a complete transition by the end of the year and Afghan authorities will be fully responsible for the country's security.

"America's war in Afghanistan will come to a responsible end," Obama told the soldiers. The visit lasted less than four hours.

The U.S. is moving away from a 13-year-conflict in Afghanistan, which is considered to be the U.S.'s longest war with a foreign country.

Over the years, the relations between Washington and Kabul have been fraught. In fact, Obama did not meet with Afghan officials who are campaigning for the June 14 election.

The U.S. President made a courtesy call to Afghanistan President Hamid Karzai. Obama praised Karzai for the efforts made by Afghan security forces and the for the country's successful conduct of its national elections. The phone call lasted for 20 minutes.

According to reports, Karzai was invited to meet the U.S. President at Bagram Airfield, but the Afghan leader refused the invitation.

Some see the invitation as Obama's last attempt to convince Karzai into signing a security agreement - a mutual security pact that would allow U.S. troops to stay in Afghan soil until 2024.

Abdul Karim Khurram, Karzai's chief of staff, said the Afghan president finds it difficult to meet Obama at Bagram Airfield, but added "President Karzai said he would warmly welcome him [Obama] if he comes to the palace" in Kabul.

Bagram Air Field is 40 km from Kabul.

In a related story, the White House accidentally included the name of a top Central Intelligence Agency (CIA) official stationed in Afghanistan during the President's visit.

The CIA official's name, identified only as 'Chief Station Officer,' was included on the list of those who will participate in a military briefing with the U.S. president.

The list was later on distributed in a 'pool report,' which refers to an individual reporter's summary of the briefing that is passed on to members of the White House press list. The list includes reporters who are not part of the trip to Bagram Airfield.

TagsU.S. President Barack Obama, Bagram Airfield, Afghanistan, Hamid Karzai, Memorial Day