With the collapse of the government in Sanaa, the United States has lost a reliable partner in its fight against Al-Qaeda in Yemen with potentially dire consequences, officials and experts said Friday.

The result could mean an emboldened Al-Qaeda in the Arabian Peninsula (AQAP), able to operate more freely amid political turmoil and without pressure from US-trained government forces.

"Terrorist groups thrive on chaos, and the threat to the West could well grow," said Daniel Benjamin, a former coordinator for counter-terrorism at the State Department.

Yemen is arguably the most important front in America's struggle against Islamist extremists, given the dangerous track record of AQAP.

The group claimed responsibility for this month's deadly attack in Paris on the staff of satirical weekly Charlie Hebdo and has been linked to more than one attempt to blow up aircraft bound for the United States.

And until now, Washington has counted on the Yemeni government to share intelligence on AQAP, support US drone strikes against the group's leaders and permit the presence of dozens of US special operations forces.

US officials admit they are unsure what will be left of its counter-terrorism program after Abdrabuh Mansur Hadi, a crucial US ally, resigned as president amid a confrontation with Shiite militia.

"Yemen has been an important partner for counter-terrorism," a defense official told AFP.

"They have not just given permission for US operations, but taken their own action on the ground... No one knows what comes next."

If a new government opted to scrap its collaboration with the Americans, then Washington would have to consider taking "unilateral" military action against AQAP, he said.

As Yemen plunged into turmoil, officials were weighing whether to evacuate the American embassy in the capital, as well as the more than 100 special operation forces advising the army.

Pentagon spokesman Rear Admiral John Kirby said the US training had not been suspended despite the crisis, while acknowledging it was a "fluid situation."

"There may have to be adjustments in the future, but we're just not there yet," Kirby said.

- More drone strikes? -

The crisis has thrown the counter-terrorism effort in Yemen "into some disarray," said Adam Schiff, a Democratic lawmaker on the House Intelligence Committee.

The prospect of conflict between Sunnis and Shiite Huthis "will only give a lot of comfort and free reign for Al-Qaeda to have a resurgence in Yemen potentially," Schiff told MSNBC.

It will be increasingly difficult to keep the US embassy open in the face of violence in Sanaa, and that would mean no more American troops assisting the Yemeni army, said Bruce Riedel, a former CIA officer who has advised several presidents.

"If the embassy closes, then the US military presence on the ground largely evaporates," said Riedel, a fellow at the Brookings Institution.

Washington then would have to rely exclusively on drone raids without assistance from local forces battling AQAP on the ground, or intelligence gleaned from captured militants, he said.

The demise of the current cooperation would mean "more, not less, drone strikes in the future," he said -- albeit without support or permission from Sanaa.

The US military and CIA have carried out more than 100 drone strikes in Yemen since 2009, as well as 15 cruise missile attacks, according to estimates from the Washington-based New America Foundation.

Some experts held out the possibility the Huthi militia that ousted the Western-backed government might opt to secretly cooperate with the Americans, as Huthis are fierce enemies of AQAP.

But the Huthis are also hostile to the United States and the West, and some former officials were skeptical that any deal could be worked out.

"We've now got a bizarre situation in which the Huthis, who are the enemy of our enemy Al-Qaeda in the Arabian Peninsula, are really not our friends and will likely inhibit our efforts to build the capacity of the Yemeni military and security forces," Benjamin said.

- Limits of US power -

Just a few months ago, President Barack Obama had cited Washington's approach in Yemen as a model for taking on Islamic extremists without having to deploy large occupying forces.

For Riedel, "it was a good strategy, but it just didn't work. It didn't work because the Middle East is unraveling. And we can't stop the unraveling."

AQAP stands to gain from the power vacuum in Yemen and any weakening of the US campaign against it, heightening the danger that the group could pull off a sophisticated attack on Western targets, he said.

"I think AQAP is going to be one of the beneficiaries of the unraveling of Yemen," Riedel predicted.