Al-Qaeda chief Ayman al-Zawahiri is a man with a price on his head, and whether he is on the move or has gone to ground, probably in Pakistan, his days are likely numbered, US experts say.

After the killing of Osama bin Laden in May in a secret raid deep in Pakistan by US elite commandos, Zawahiri took over at the head of the organization behind the September 11, 2001 attacks.

Elimination of the co-founder of the Islamic militant organization would be a devastating blow to Al-Qaeda's central command, and Zawahiri is being relentlessly pursued by the United States, say experts.

"A missile could strike him anywhere, day or night, at any time," said psychiatrist Marc Sageman, a former CIA agent in Pakistan and author of "Leaderless Jihad."

"If he waits too long in the same place he runs the risk of being spotted. But if he moves it's worse, he becomes even more vulnerable. It's an untenable position."

It was by slowly piecing together the trail of his only authorized contact that bin Laden was pinpointed and then killed during the US helicopter-borne raid on his compound in Abbottabad, Pakistan.

That precedent should haunt Zawahiri's days and nights, said Thomas Hegghammer, a terrorism specialist at the Norwegian Institute for Defense Studies.

"Bin Laden was very cautious but he had to maintain a certain amount of contact with the organization. And it got him killed," Hegghammer said during a conference in Washington on Al-Qaeda after bin Laden.

"I think al-Zawahiri is doing the same, with even a lower profile, being more careful of who he is talking to," he said.

Yet speaking to contacts is vital if he wants to retain some influence. "And every time he is doing it, he is taking a life-threatening risk. He has a choice: fading away or risking his life," Hegghammer added.

"He knows the CIA is working hard on him. He is on borrowed time. They will take him out. Tomorrow, in two months or in two years. But they will get him, too," he said.

Unquestionably, the most effective weapon in the US war against Al-Qaeda has been the CIA's secret -- and thus never officially acknowledged -- drone campaign targeting key jihadists in Pakistan.

But in a worrying setback for the drone program, US relations with Islamabad have plummeted since the Bin Laden raid, most recently over the killing of 24 Pakistani troops in a US air strike near the Afghan border.

Under Pakistani orders, the United States over the weekend vacated Pakistan's Shamsi air base where the CIA's drones were reportedly based.

Experts say the loss of the Shamsi base is not insurmountable, as drones can be flown from Afghanistan.

But the drone strikes, once a daily occurrence, appear to have been on hold since mid-November, and it is unclear when they will resume, said Andrew Liebovitch, an expert at the New America Foundation.

"US and Pakistani officials have said that Shamsi was being used mostly for maintenance and support operations, and that operations were shifted away from Shamsi following the May raid that killed Osama bin Laden," he said.

"What poses more of a threat to drone operations is the possibility that Pakistani forces will shoot down drones operating over Pakistani territory," Liebovitch added.

But drones are still the handiest US weapon to deploy against Zawahiri, should he be located, and Douglas Lute, Obama's principal advisor on Afghanistan and Pakistan, insists US strategy will not change.

"We need to go for the KO (knock-out) punch. I would not adjust today programs that are designed for the KO punch. I'm not going to shift any gear in the next six months when we have the chance of a lifetime," Lute said.

According to Brian Fishman, an expert at the New America Foundation and the Combating Terrorism Center at US military academy West Point: "Al-Zawahiri is going to get killed someday. I don't know when. I didn't expect bin Laden to last as long as he did."

"If you are al-Zawahiri you know what is going on. You spend all your time and energy surviving. Survive to speak on the Internet another day," he said. "You don't know who you can trust, you watch over your shoulder, see traitors everywhere."

The US hot pursuit has the Al-Qaeda chief "on the run," Fishman added.

"He only can speak on camera, for broadcast on jihadi websites. He can't take part in any large plot, it would kill him," he said.