Four months after succeeding Osama bin Laden at the head of Al Qaeda, Ayman Al Zawahiri is spreading jihadist propaganda over the Internet but must mainly be preoccupied with his own survival, experts say.

On June 16, six weeks after Al Qaeda founder bin Laden was killed in a US raid, 59-year-old Egyptian Zawahiri was chosen to replace him as "commander-in-chief" of the Islamist militant group, a post previously unknown to the outside world.

Since then some jihadist groups, such as the Yemeni-based Al-Qaeda in the Arabian Peninsula (AQAP), have pledged allegiance.

Other factions which had previously followed bin Laden's calls have been content to welcome Zawahiri's appointment.

Jean-Pierre Filiu, professor at the Paris Institute of Political Studiesand author of a history of Al Qaeda, sees the Islamist group's actions over the last four months as "purely defensive."

"They are presiding over a jihadist capital which is diminishing," he told AFP.

Nowhere is it making any real progress, he added. While AQAP may be taking some towns, like Zinjibar in Yemen, that is "mainly due to the chaos reigning in the country. And even in this case it doesn't amount to a victory for Zawahiri".

In the face of the maelstrom of the Arab Spring, which has seen regional governments fall through people power -- though sometimes with a large helping hand from NATO and the West -- Zawahiri has hailed the successive fall of despotic regimes, doing so via Islamist websites.

Last month the veteran of the Egyptian opposition forces called on Libyans to found an Islamic regime and for Algerians to revolt.

In July he hailed the Syrian "mujahedeen."

However Filiu said that his pronouncements have fallen "practically unnoticed in the Western media. Compared to bin Laden's threats it's negligible."

Dominique Thomas, specialist on Islamism at another French political science school, the EHESS, said Zawahiri was sending messages to affiliated groups, "trying to keep himself above the crowd," with the main message that the Arab revolutionaries should create Islamic states and not allow the West to "steal their revolutions".

But Zawahiri, a permanent target for US drones' Hellfire missiles with a $25 million bounty on his head, is in no position to mount or coordinate major operations. His imperative is survival.

"The pressure from the Americans is enormous," Thomas stressed.

When US-Yemeni Imam Anwar Al-Aulaqi was killed in Yemen in September "he had not taken the threat seriously".

In the mountainous, inaccessible Afghanistan-Pakistan border region where Zawahiri is believed to be hiding out, "there are drone attacks almost every day," Thomas added.

For Douglas Lute, US President Barack Obama's main adviser on Afghan and Pakistan affairs, "Al Qaeda is in uncharted waters after the death of Bin Laden."

"They never had a succession process, this is a period of turbulence for this organisation which is our arch enemy," he added.

"In this succession period there are three to five key senior leaders in Al Qaeda that if removed from the battlefield would seriously jeopardize Al Qaeda's capacity to regenerate and therefore move us decidedly further toward defeat."

Zawahiri knows whose name is at the top of that list.