A Taliban suicide bomber targeting NATO medical trainers infiltrated Kabul's main military hospital on Saturday and blew himself up in a tent full of Afghan medical students eating lunch, killing six and wounding 23.
No foreign medical doctors or nurses were among the dead or wounded, Afghan and NATO officials said.
The blast, which thundered across the capital, came as the Taliban have stepped up attacks as part of a spring offensive against NATO, Afghan government installations and officials. Insurgents also have promised revenge attacks after the U.S. killing of Osama bin Laden.
Taliban spokesman Zabiullah Mujahid claimed responsibility for the attack, saying it was targeting foreign trainers and Afghan doctors who work with them. He claimed two bombers took part, but Defense Ministry spokesman Gen. Mohammed Zaher Azimi spokesman said only one attacker was involved and only one blast was heard at the Mohammad Daud Khan military hospital.
The hospital is the largest in Kabul with 400 beds, and its grounds contain numerous buildings and small parks. Afghan army and police stepped up patrols throughout the capital for fear of more attacks.
Azimi said all the dead and wounded were civilians and university students.
He added that "tents are used because there is not enough space in the hospital. This area is used by university students for training and eating meals."
There have been many reports in recent weeks that insurgents were planning suicide attacks and other acts of violence inside the capital. Such attacks have been relatively rare and usually number a couple a month.
The ability of the attacker to get inside the heavily guarded hospital raised fresh concerns about possible infiltration of Afghan security forces. The facility is in one of Kabul's most secure neighborhoods and close to NATO headquarters, the U.S. Embassy and other diplomatic facilities. Security checks are stringent and all visitors are searched.
Mujahid said that was part of the Taliban strategy against the government. "The mujaheddin are able to infiltrate into the ranks of the enemy and using opportunities are able to attack," he said.
It was unclear if the bomber was in fact a member of the staff, but there have been a number of incidents where Afghan security forces have turned on their trainers and killed them. On April 27, a veteran Afghan military pilot said to be distressed over his personal finances opened fire at Kabul airport, killing eight U.S. troops and an American civilian contractor.
In the most embarrassing infiltration of a government facility, a Taliban militant opened fire inside the Afghan Defense Ministry on April 18, killing two Afghan soldiers. At the time, the Taliban said one of their agents who was also an army officer planned the attack to coincide with a visit of the French defense minister, who was not in the ministry at the time.
NATO has been expecting the Taliban to stage a series of spectacular and complex attacks, and the group has already carried out a number of them around the country. Afghanistan's war usually follows an annual cycle, with fighting increasing in the spring and summer as insurgents pour over the mountainous border from Pakistan.
The ferocity of the Taliban's spring offensive will help determine whether the surge of more than 30,000 additional U.S. troops that President Barack Obama announced in December 2009 succeeded in arresting the insurgency. It could also affect the size of Obama's planned drawdown of U.S. troops in July. Gen. David Petraeus, the top U.S. and NATO commander in Afghanistan, has said the size of the withdrawal will depend on conditions on the ground.
The alliance has committed itself to handing over control of security in the country to Afghans by 2014.