KABUL - Militants detonated a massive car bomb and seized a building outside a main US and NATO military base in Afghanistan's Bagram on Wednesday morning, pushing security forces to respond, officials and witnesses said.

"A car bomb explosion occurred at 5:50 am local time Wednesday in Bagram district. We still cannot provide details on causalities. There is fear of possible causalities," Nasrat Rahimi, spokesman of the Afghan Interior Ministry, told Xinhua.

Further details will be released as appropriate, Rahimi added.

"We heard a huge blast in Janqadam locality in Bagram district after militants detonated a huge car bomb. A second group of militants seized a health clinic's building, clashing with security forces and firing rocket propelled grenades (RPGs) onto the Bagram base," a witness told local Arman FM radio.

The whole place has now been sealed off by US and Afghan security forces, the witness added.

The blast caused a plume of gray smoke to rise above the scene.

Military helicopters were hovering over the site.

Located in Bagram district of eastern Parwan province, the Bagram military base, some 50 km north of Afghan capital of Kabul, has been serving as a main US and NATO military base in Afghanistan over the past 18 years.

No group has claimed responsibility for the attack yet.