"They are considered dangerous and are suspected terrorists. That is why they were detained initially," a U.S. military spokeswoman, Lt. Cindy Moore, told The Associated Press.
The four are Arabs from Syria, Saudi Arabia, Kuwait and Libya, said Kaber Ahmad, the government chief in Bagram town, which is adjacent to the vast U.S. base by the same name, and whose security forces are helping in the search.
"Coalition forces, police and Afghan troops have surrounded several villages near the base," he said. "The coalition has distributed photos of the four. They all have short hair, long beards and are wearing yellow prison clothes."
Moore declined to identify the four or elaborate on why they were being held. Another military spokesperson, Lt. Col. Jerry O'Hara, described them only as "enemy combatants."
He said it was the first time anyone has broken out of Bagram's heavily guarded detention facility. The base is home to thousands of U.S. and coalition soldiers.
U.S. helicopters, American troops on the ground and Afghan forces were scouring the area around Bagram, an hour's drive north of Kabul, for the four suspects who went missing around dawn, she said.
About 500 prisoners are being held by U.S. forces in Afghanistan, most of them at Bagram, O'Hara said.
The escape comes after allegations that U.S. military personnel at Bagram and at other detention facilities have abused inmates. The U.S. military has said it would not tolerate any maltreatment.