WASHINGTON (CNN) -- Two U.S. military Humvees are missing after suspected Taliban militants attacked a transport convoy carrying equipment destined for U.S. troops in Afghanistan, U.S. military and local officials said Tuesday.
U.S. military officials in Afghanistan did not have much detail about Monday's attack in northwestern Pakistan, but said the supply convoy was carrying at least two Humvees and numerous water tank trailers.
As many as 70 armed militants attacked two convoys within 30 minutes at the Khyber Pass, a mountain pass that links Pakistan and Afghanistan, local officials said. The pass is located in Khyber, one of seven semiautonomous tribal agencies along the Afghan border.
U.S. military spokesman Maj. John Redfield said he did not know how many transport trucks were stolen, but some reports said militants took as many as 13.
U.S. military officials are working with Pakistani officials to track down the missing trucks. Local officials said it's believed tribal leaders in the region will hold talks with the militants to try to secure the return of the trucks and their supplies.
Khyber Agency officials said some of the trucks seized in one of the attacks were carrying wheat to Afghanistan as part of a United Nations World Food Programme convoy, and one truck contained Humvees destined for the U.S.-led coalition in Afghanistan.
Redfield said the convoy had more than 13 transport trucks in it. The Humvees were being carried on a flatbed truck and the water tanks were in container trucks, he explained.
No U.S. troops were accompanying the convoy, Redfield said. It was operated by a commercial contract company and was being guarded by Pakistani Frontier Corps troops when the attack occurred Monday, he said.
Authorities dispatched two helicopter gunships, which fired on the raiding militants killing one person and wounding another, but could not foil the hijacking, local officials said.
The ground route through Khyber Pass is one of many used by U.S.-hired contractors to ferry equipment into the Afghanistan war zone from Pakistan.
Over the past year, these ground routes have come under numerous attacks by Taliban and al Qaeda militants, officials said.
One raid earlier this year resulted in the hijacking of a truck carrying a U.S. Army helicopter engine.
The Pakistani central government has little control in the area, and it is believed to be a haven for militants.
The United States uses Pakistan as a supply route because of its major sea port in Karachi. Afghanistan is a land-locked country and the fighting there makes it more difficult to fly in supplies.
A suicide bomber struck outside a stadium in northwest Pakistan on Tuesday evening, killing three people and wounding 11 others, the provincial information minister, Mian Iftikhar Hussain, said.
The blast occurred outside Qayyum Stadium's front gate, where a closing ceremony for the Inter-Provincial Games was being held, police said.
The ceremony for the sports tournament was just ending when the blast went off, North West Frontier Province police Inspector General Malik Naveed said.
Most people had already left the stadium at the time of the attack, he said.
In addition to the three dead, the bomber was also killed, police said.
Qayyum Stadium is in the heart of Peshawar, the capital of the North West Frontier Province on the country's border with Afghanistan. The region is rife with Islamic extremists.
CNN's Zein Basravi contributed to this report