Militants killed a driver of a container truck carrying supplies for NATO in Pakistan's tribal region on Monday, a government official told CNN.

Three gunmen in a car opened fire on the truck in the Jamrud area of Khyber Agency, a district in Pakistan's tribal region bordering Afghanistan, said Bakhtiyar Khan, a government official in the district.

Khan said three members of the truck crew were also injured in the incident as the vehicle moved towards the Pakistan-Afghanistan border.

There was no immediate claim of responsibility for the attack. Several anti-American militant groups, including the Taliban, are active in the region.

Pakistan reopened routes into Afghanistan a month ago after closing it to NATO traffic in anger at the killing of 24 Pakistani troops by coalition forces.

The Pakistani Taliban threatened to attack NATO trucks when the border reopened, saying the supplies are used to target its members fighting against occupation in Afghanistan.

Taliban have attacked NATO supplies in the past, and killed drivers and other crew.

On July 5, trucks carrying supplies to NATO troops crossed from Pakistan into Afghanistan for the first time in seven months, officials said.

Because Afghanistan is landlocked, many supplies for NATO-led troops fighting Islamic militants have to be trucked in from Pakistan.

On July 3, Islamabad decided to reopen the crucial supply routes shut down on November 27, a day after coalition forces mistakenly killed the 24 Pakistani troops.

The Pakistani routes offer a shorter and more direct path than ones through Russia and other countries which NATO was forced to use when the border was closed.

It has cost the U.S. an extra $100 million a month to use the alternative northern routes.