Islamabad, Pakistan ? An Afghan warlord with links to Iran and Pakistan’s powerful spy agency has called for a holy war against the United States and Britain and vowed to rally like-minded radicals, such as al-Qaida, under one banner to do battle.

“I invite all the believers to be united and to be ready for war to liberate your country from the foreign oppressors,” Gulbuddin Hekmatyar said in a handwritten letter circulated in Afghanistan and to some of his followers in Pakistan. A copy of the Pashtu-language letter was obtained by The Associated Press on Thursday.

A British member of the International Security Assistance Forces, center in maroon beret, helps members of the First Battalion of the Afghan National Guard in a riot drill in Kabul, Afghanistan. The international forces have been training the guard unit, which will be protecting the upcoming loya jirga meeting to choose a new Afghan government.

Washington clearly takes Hekmatyar seriously.

Earlier this month, U.S. officials said one of their unmanned Predator drone aircraft fired a missile at Hekmatyar north of the Afghan capital, Kabul. He survived and said in his letter that reports of deaths among his followers were false.

This week, hundreds of British troops launched Operation Buzzard to patrol near the Pakistani border where they believe Hekmatyar may be active.

The targets of Operation Buzzard are Taliban, al-Qaida and “pan-Islamists,” said a British military official, speaking on condition of anonymity. Asked to identify a “pan-Islamist,” he said: “Gulbuddin Hekmatyar would be one of them.”

Hekmatyar could help al-Qaida regain a foothold in Afghanistan, the official said.

The new British deployment will last for several weeks and cover plains south and east of the city of Khost, near the Pakistani border, where U.S. intelligence says Hekmatyar has a following among Afghanistan’s majority Pashtun ethnic group.

Hamid Gul, a former chief of Pakistan’s spy agency, said Hekmatyar should not be underestimated. Gul was closely allied to Hekmatyar during the 1980s Soviet invasion of Afghanistan and during the bitter factional fighting that followed the collapse of the pro-Moscow regime in Kabul in 1992.

“They should be afraid of Hekmatyar. He is a hard-liner, who has a large following,” said Gul, whose intelligence agency funneled millions of dollars in weapons to Hekmatyar during the 1980s war.

In 1992, when the U.S.-backed Islamic insurgents took power in Kabul and turned their guns on each other, Gul continued to support Hekmatyar.

As prime minister of the feuding government, Hekmatyar fought bitterly with Defense Minister Ahmed Shah Massood, killed last September in a suicide bombing. Hekmatyar pounded Kabul with thousands of rockets until 1996, when he finally made peace with Massood and took power.

Within five months, the Taliban had taken over, forcing Massood and Hekmatyar to flee.