Four U.S. troops have been killed in an attack in Afghanistan which came just hours after Washington announced plans to open talks with the Taliban, Belfast Telegraph reported.

Defense officials said the four were killed by indirect fire, likely a mortar or rocket, at Bagram Air Base.

It came after sources in the Obama administration said a Taliban office is to be opened in the Qatari capital of Doha - the first step towards the ultimate U.S.-Afghan goal of a full Taliban renouncement of links with al Qaida. Formal meetings will begin within days.

The attack also came as U.S. and allied forces formally handed over control of the country's security to the Afghan army and police in a ceremony in Kabul.

The transition to Afghan-led security means the U.S.-led coalition will not be directly carrying the fight to the insurgency, but they will advise and back up the Afghan forces as needed with air support and medical evacuations.