KABUL, Afghanistan – In a brazen attack in Kabul’s most secure district, a Taliban suicide bomber on a mission to target foreign-run medical teams infiltrated the capital’s main military hospital and killed at least six Afghan medical students and wounded 23 others.

The bombing was a blow to Afghan and NATO forces that have sharply expanded checkpoints and security cordons in the Afghan capital as the Taliban intensifies their attacks ahead of a planned U.S. drawdown in July. It also indicated the insurgency may now be favoring terror-style tactics rather than larger traditional battles.

The attack came as a related group — the Taliban in neighboring Pakistan — claimed responsibility for two attacks on tankers carrying fuel for NATO forces in Afghanistan.

One assault was a twin strike: the first blast damaging a tanker and then a second explosion that killed 15 people trying to siphon fuel from the disabled vehicle in northwestern Pakistan. Later, 14 NATO tankers were gutted in a bombing at a nearby border town, but no one was hurt.

The Pakistani Taliban is a network of militant groups that is distinct from the Afghan Taliban, but linked by ideology. They help shelter militants who conduct cross-border attacks into Afghanistan.

In the Kabul attack, no foreign medical doctors or nurses were among the dead or wounded, Afghan and NATO officials said. There are a number of military doctors and nurses from various NATO countries at the hospital as part of the alliance’s mission to train Afghan forces.

All those killed were eating lunch inside a tent used by medical students for meals.

The bombing was condemned by President Hamid Karzai and NATO. The United Nations called it a violation of “international humanitarian law.”

The blast came as the Taliban stepped up their attacks as part of their spring offensive against NATO, Afghan government installations and officials. Insurgents also have promised revenge attacks after the U.S. killing of al-Qaida leader Osama bin Laden in Pakistan earlier this month.

The effectiveness of the Taliban’s campaign will in part determine the size of President Obama’s planned drawdown of American troops. He has said its size will depend on conditions on the ground.