ISLAMABAD - President Barack Obama's national security adviser declared Ayman al-Zawahiri to be the world's "No. 1 terrorist" on Sunday, but said al-Qaida's long-standing deputy lacked Osama bin Laden's charismatic appeal.

A week after bin Laden's death and with no successor yet named, the assessment will heighten speculation that al-Qaida is heading for a leadership battle.

"Our assessment is that he is not anywhere near the leader Osama bin Laden was," Tom Donilon said.

He also said there was no evidence yet to suggest that the Pakistani authorities were aware bin Laden had set up home in Abbotabad only 50 kilometres from the capital Islamabad.

Al-Zawahiri, an Egyptian-born surgeon, is the man in line to take over, according to al-Qaida's own rules of succession, but intelligence officials and analysts believe his abrasive style has made him unpopular within the organization.

Other possible contenders in a succession battle include Abu Yahya al-Libi and Atiyah abd al-Rahman, two Libyans seen as increasingly important.

Anwar al-Awlaki, the U.S.-born radical cleric famous for his YouTube appeals to English-speaking converts from his base in Yemen, has also been mooted. However, his high profile in the West is not matched by his status within the terrorist network. He has little combat experience, no links to bin Laden and is a member of al-Qaida in the Arabian Peninsula - an offshoot of the core leadership.

U.S. officials have described the succession as an "open question."

"Al-Zawahiri is obviously the presumed successor, but there are strong indications he is not popular within certain circles of the group," said a senior CIA official during a briefing to reporters this weekend.

"It is, of course, anathema to al-Qaida to hold free and fair elections. If free and fair elections were conducted, Zawahiri would most likely have a fight on his hands. He has repeatedly promoted jihad in Egypt above other objectives, costing him traction within other elements of al-Qaida, particularly those drawn from Saudi Arabia, Yemen and the Gulf states. And he lacks the charisma of bin Laden."

Hasan Askari Rizvi, a Pakistani analyst, said it would take time for al-Qaida's dispersed leadership to select a new emir - or paramount leader - particularly as U.S. surveillance would be in a state of high alert.

"What usually happens is that the leader anoints his successor which Osama was unable to do because he had shut down a lot of channels of communication in hiding," he said. "So there is a possibility of a power struggle."

For now, al-Qaida has limited itself to a statement confirming bin Laden's death with no mention of a new leader. It said bin Laden's blood "will be a curse that will chase the Americans and their agents."