Brussels: NATO's chief piled pressure on Pakistan on Friday to step up the fight against "terrorists" enjoying safe havens in the border region with Afghanistan.

Amid growing US pressure for Pakistan to take action against Al-Qaeda-linked extremists, NATO Secretary General Anders Fogh Rasmussen called for a "positive engagement" from Islamabad to ensure stability in Afghanistan.

"We encourage the Pakistani military and the Pakistani government to do its utmost to fight extremism and terrorism in the border region," Rasmussen said at a defence forum hosted by the European Policy Centre think tank.

"It is really a security problem for our troops in Afghanistan that terrorists have safe havens, and that's a fact, in Pakistan," he said. "We have to deal with that and it's in our mutual interest to deal with that."

"That's a reason why we have conveyed that clear message to Pakistan authorities."

The Pakistani government and opposition leaders on Thursday closed ranks against increasing US pressure for action against the Al-Qaeda-linked Haqqani network, refusing to be pressured into doing more in the war on terror.

In an unprecedented condemnation of Pakistan, the outgoing head of the US military, Admiral Mike Mullen, last week accused the country of "exporting" violent extremism to Afghanistan through proxies.

Mullen also charged that the country's main intelligence agency, ISI, was actively supporting the Al-Qaeda-linked Haqqani network blamed for an assault on the US embassy in Kabul this month.

There are 140,000 NATO-led foreign forces in Afghanistan, some 100,000 of them from the United States, fighting a Taliban-led insurgency.