US military and intelligence agencies have launched an elaborate manhunt in Libya against the militants suspected of staging the most serious assault on an American diplomatic mission in decades, officials and experts said Friday.

With hi-tech weaponry and surveillance tools, the Pentagon and US spy services have turned their attention to finding those who laid siege to the American consulate in Benghazi on Tuesday, employing methods honed over years in Afghanistan, Pakistan, Yemen and elsewhere.

"There's an intense focus" on finding the attackers, a US defense official, who spoke on condition of anonymity, told AFP.

Chaotic conditions in Libya after the fall of dictator Moamer Kadhafi will complicate the search, and US officials are still grappling with questions about exactly who was behind the attack -- amid speculation the assault may or may not have been pre-planned and linked to the Al-Qaeda network.

But Washington enjoys friendly ties with the new government in Libya, which allows the United States to fly drones over the country and will be ready to cooperate in any operation aimed at suspected militants, experts and former officials said.

President Barack Obama issued explicit warnings in the wake of the Benghazi attack, making clear that Washington would be targeting the gunmen whose assault killed the US ambassador and three of his staff.

"I want people around the world to hear me, to all those who would do us harm: no act of terror will go unpunished," Obama said on Thursday.

So-called "targeted killings" with unmanned drones in the sky and night raids by elite US special operations forces have become a hallmark of Obama's presidency, despite diplomatic tensions and the resulting civilian casualties.

Obama has not hesitated to use lethal, covert force to retaliate against those who Washington deems imminent terror threats, approving a risky helicopter raid into Pakistan last year that killed America's arch-foe, Al-Qaeda chief Osama bin Laden.

"What we're likely to see in the next few days is a very aggressive approach to try to deal with these fringe groups," Brian Katulis, senior fellow at the Center for American Progress, told reporters earlier this week.

"I think that's been an overall feature of the Obama administration's policy in the Middle East. Vigilance on terrorist threats whether it be in Pakistan or in Yemen and now in Libya.

"And I would expect some form of a response under way now."

The Americans, however, may choose to have Libyan forces take a visible lead on the ground to avoid any appearance of trampling on an Islamic country's sovereignty, analysts said.

In Afghanistan, helicopter-borne US special forces teams have carried out thousands of night raids since Obama entered the White House in 2009, with the Americans seeking to catch Taliban fighters by surprise and "remove" mid-level commanders from the battlefield.

The raids have been scaled back amid complaints from Kabul that the assaults sow anti-American sentiment and inflict innocent civilian casualties.

In Pakistan's tribal areas, armed Reaper drones have been the weapon of choice for the Obama administration as it seeks to kill Al-Qaeda, Taliban and other Islamist extremist targets.

The United States has stepped up operations against Al-Qaeda militants in Yemen also, including the use of drones, as extremists have sought to exploit political unrest there.

The drone bombing campaign in Pakistan, which started on a lesser scale under former president George W. Bush, has dramatically expanded under Obama and reportedly now targets mid-level Taliban figures as well as more senior Al-Qaeda operatives.

Obama has authorized an estimated 283 strikes in Pakistan, six times the number during Bush's eight-year tenure, according to author and terrorism expert Peter Bergen of the Washington-based New America Foundation, who has written books about the hunt for bin Laden.

The emphasis on "targeted killings" and stealthy raids by special forces has led to a close working relationship between the US military and intelligence agencies, producing a new generation of officers and CIA agents with expertise in how to conduct manhunts.