Pakistani truckers are refusing to haul vital supplies to NATO and U.S. forces in Afghanistan because of mounting attacks along the main route, a transporters association said Monday.

Pakistani troops recently began escorting convoys through the Khyber Pass to the border to protect them from Taliban ambushes. Western military officials insist their Afghan operations are not at risk.

However, suspected militants have pulled off a series of bold raids on depots near the city of Peshawar in recent weeks, killing several guards and burning hundreds of vehicles, including scores of Humvees.

Shakirullah Afridi, the president of the Khyber Transport Union, said Monday its members had been using some 3,500 trucks and trailers to carry fuel, food and other supplies to Afghanistan.

He said all were boycotting work carrying military supplies — and that no offer of improved terms and security would persuade them to risk their lives or equipment again.

"If all the countries of NATO cannot control the situation in Afghanistan, how can escorts from the Frontier Corps ensure our safety?" Afridi said, referring to the paramilitary force that guard the convoys.

Kifayatullah Khan, the manager of the Port World Terminal, where a guard was shot dead during a recent attack, confirmed a shortage of trucks but said he was trying to persuade the transport firms to return.

A spokesman for the NATO-led force in Afghanistan played down the boycott threat and said it did not deal with Afridi's association.

"If suddenly 70 percent of our stuff isn't reaching us, we'd know about it, and that's not the case," Lt. Cmdr. James Gater said. "There is no indication to us that there is a disruption to our supply lines at this stage."

Bakhtiar Khan, a government official in the Khyber region, said a convoy of 191 vehicles carrying military supplies crossed into Afghanistan on Monday.

Up to 75 percent of the supplies for Western forces in the landlocked country pass through Pakistan after being unloaded from ships at the Arabian sea port of Karachi.

Most of the material passes through Peshawar, which lies on the edge of Pakistan's lawless tribal regions where Taliban militants hold increasing sway and Osama bin Laden and other top al-Qaida leaders are believed to be hiding.

NATO says it is investigating alternative supply routes through Central Asian nations to reach its forces, but has acknowledged that they are more expensive.

The U.S. military plans to send thousands more troops to Afghanistan in the coming months.