WASHINGTON, Jan. 24 (Xinhua) -- Four U.S. citizens were killed and two others injured in a weekend attack at a hotel in Kabul, the capital city of Afghanistan, the U.S. State Department said Wednesday.

"The United States strongly condemns the attack on January 20 at the Intercontinental Hotel in Kabul. We can confirm that there were four U.S. citizens killed and two injured," said U.S. State Department Spokesperson Heather Nauert in a statement.

At least 18 people have been killed in the deadly attack, including 14 foreign nationals, Afghan authorities said Sunday. All five attackers were also killed during the gunfight with Afghan security forces.

A 12-hour siege following Saturday night's attack ended on Sunday.Afghan officials have said an investigation is underway to find out how the attackers got into the building so easily.