This story was filed by CBS News' Farhan Bokhari in Islamabad.
Taliban militants driven out of Pakistan's northern Swat valley in a 2009 military campaign are threatening a comeback.
A video obtained by CBS News' Sami Yousafzai shows Mullah Fazlullah, the former leader of the Taliban in Swat, addressing a group of followers -- described as suicide bombers -- and urging them to target Pakistan's government and the country's security forces.
The video, which shows Fazlullah (center in photo) walking with an unexplained limp, comes after reports in May which suggested the militant commander had been killed in a strike along the Pakistan-Afghanistan border.
"I am asking my Fidayeen (suicide bombers) to target two kinds of people; the military and its supporters, and the nationalists and their leadership," Fazlullah tells the militants before him.
"Make them your targets," he says in what appears to be his first-ever video appearance. Fazlullah goes on to tell his assembled followers that Islam would even permit the accidental taking of innocent lives in the effort to slay the enemy.
A senior Western defense official in Islamabad tells CBS News the video is "a serious development."
"We can now expect a major attack," said the official, who spoke on condition of anonymity. "This threat should not be seen as an idle one."
A Pakistani intelligence official who saw the video in the northern city of Peshawar said the background scenery appears to be from a valley somewhere in the lawless tribal regions which straddle the Afghan-Pakistan border.
According to the official, the visual clues suggest Fazlullah may already have assembled a group of hardcore militants in a sparsely populated region and is positioning himself to launch a string of fresh attacks, possibly in both Afghanistan and Pakistan.
"Neither in Afghanistan nor in Pakistan can security be taken for granted. We have to be on our toes and remain very vigilant," said the intelligence official, adding that "these militants know that the U.S. is somewhat vulnerable in Afghanistan and is far from a clear victory, so from their point of view, this may be a good time to attack U.S. and other Western soldiers."
Hasan Askari Rizvi, a respected Pakistani commentator on security affairs, tells CBS that Fazlullah's apparent return raises a number of serious questions for the U.S. and Pakistan, ranging from an intelligence failure in tracking him down after last years' campaign, to a very real new challenge to the military's effort to consolidate power in Swat.
"I believe someone should be asked to explain how this could have happened when a man was believed to be dead not too long ago, and he is now openly and daringly threatening us all," said Rizvi.
"Any further attacks could be very disruptive, because they will only demonstrate that the Taliban defeated in Swat have returned to fight for their cause," added Rizvi.
A senior government official in Swat, who also spoke to CBS News on condition that he remain unnamed, said Fazlullah's threat could force a review of cultural events planned by the government to revive the once festive holiday atmosphere in the picturesque Swat valley.
"We simply cannot risk having musical shows or other similar activities. If Fazlullah and his men are around, they will be looking for exactly such targets," said the official.
Before Swat was completely taken over by the Taliban, it was a prime vacation destination for Pakistanis and others from the region.