A suicide bomber killed at least six people and wounded 23 more at a military hospital in a heavily-guarded area of the Afghan capital Kabul on Saturday, the Defence Ministry said in a statement.

The bomber struck at 12:30 p.m. local time (0900 GMT) in the cafeteria of the hospital where medical students were eating lunch, said Mohammad Zahir, head of the police crime investigation unit. He added the dead were students.

The hospital is located in a high-security area near the U.S. embassy and where several other foreign embassies and international organisations operate.

Taliban spokesman Zabihullah Mujahid said in an emailed statement that the group had sent two suicide bombers into the hospital and killed many. The Taliban launched a long-awaited "spring offensive" last month.

Despite escalating violence in the 10th year of an increasingly unpopular war in Afghanistan, Kabul is relatively peaceful. In early April, a suicide bomber attacked an Afghan army bus on the outskirts of the capital, wounding 10 soldiers and civilians.

The Taliban, which often inflates the number of casualties it inflicts, vowed last month to carry out attacks, including suicide bombings, on foreign and Afghan troops and government officials.

Despite the presence of up to 150,000 foreign troops, violence across Afghanistan is at its worst since the 2001 overthrow of the Taliban government by U.S.-backed Afghan forces, and last year saw record casualties on both sides.