Pakistan has told Washington that US forces must seal the Afghan border in the event of any offensive against the Al-Qaeda-linked Haqqani network in North Waziristan, an official said Saturday.

The Haqqanis, blamed for some of the deadliest attacks in Afghanistan and whose leaders are understood to be based in the Pakistani tribal district, is one of the thorniest issues between Islamabad and Washington.

"The Americans have been repeatedly told that they will have to seal off the border on the Afghan side whenever an operation is launched in North Waziristan," a senior Pakistani security official told AFP.

Without protecting the porous, mountainous border, militants would simply escape into Afghanistan, where Pakistan has no writ, the official explained.

He claimed that Americans have "never been encouraging on this point" and accused them of failing to seal the border when operations were planned twice before in North Waziristan.

On August 3, The Wall Street Journal reported that Pakistani and US officials were considering joint counter-terrorism campaigns in Afghanistan and Pakistan against the Haqqanis and Taliban fighters who attack Pakistan.

The paper said the campaigns would mark an upturn in cooperation after more than a year of rancorous relations and stamp out major threats facing each country.

Pakistani officials later denied any agreement with the United States for a joint operation in North Waziristan, and said "routine" actions on each side of the border "should not be mistaken for 'joint operations'".

Washington has long demanded that Pakistan take action against the Haqqanis, whom the United States accused of attacking the US embassy in Kabul last September and acting like the "veritable arm" of Pakistani intelligence.

Pakistan has in turn demanded that Afghan and US forces to do more to stop Pakistani Taliban crossing the Afghan border to relaunch attacks on its forces.

The senior official told AFP that Pakistan had been able to "speak their heart and mind" on the issue during last week's visit to Washington by the Inter-Services Intelligence (ISI) chief.

Lieutenant General Zaheer ul-Islam, in the first such visit for a year, and CIA director David Petraeus discussed some of the most intractable issues on both sides that have fractured the anti-terror alliance.

"The Americans were clearly told that Pakistan will not allow American boots on its soil for any operation and whenever an offensive is launched, it will be done by us," the official told AFP.

"We told the Americans that it is simply not possible for Pakistan to launch a fresh offensive in North Waziristan at the moment because it will have a very negative impact," he added.

Some analysts question to what extent Pakistan can win a full-on battle against the disciplined Haqqani faction, particularly when its troops are already over-stretched against local Taliban elsewhere in the northwest.

Islam gave the CIA in Washington "two loud and clear messages," said the official -- no American boots on Pakistani soil and that US drone strikes on Islamist militants, which Islamabad brands a violation of its sovereignty, must stop.

Many in Pakistan accuse the Americans of demanding a Pakistani offensive to mask their own failings in the 10-year war in Afghanistan.

"I will be surprised if Pakistan agrees to a joint operation," said political analyst Hasan Askari.