A taxi loaded with explosives blew up near a U.S. airfield in eastern Afghanistan, killing three foreigners and one man suspected of links to the Taliban and al Qaeda. They were believed to have been planning a terrorist attack, an Afghan military commander said today.

A man from Yemen, two Pakistanis and an Afghan who was a former intelligence officer for the ousted Taliban, were killed in the blast near the town of Khost, according to Gen. Khail Baz Sherzai, the regional military commander.

The four were testing a remote-control device when they blew up the car, about 200 yards from an airfield used by U.S. forces, Sherzai said.

The former Taliban intelligence officer, Zarat Khan, had ties to al Qaeda, Sherzai said. Afghan authorities arrested some members of his family after the blast and turned them over to U.S.-led coalition forces for questioning, Sherzai said.

Col. Roger King, a U.S. Army spokesman, said tonight that he was aware of a car blast near Khost, but had no information about its proximity to any U.S. military installation.

The men drove the taxi across the border from Pakistan, Sherzai said. They parked the car, which was loaded with mines and explosives, at Khan's home near Khost, he said. The blast destroyed Khan's two-story home and injured a woman nearby, Sherzai said.

Afghan officials had been searching for Khan, who is believed to have been involved recently in firing rockets at a U.S. base in the area, Sherzai said.

The explosion was the latest sign that the Taliban is reorganizing to oppose the U.S. military presence and destabilize the fragile Afghan government.

The involvement of a Yemen national and two Pakistanis in the car blast increases "the possibility of involvement by al Qaeda," King said.