Adam Gadahn, an American born al-Qaida militant, has called for attacks on US diplomats and embassies across the Arab world.

Hailing the killer of US ambassador Christopher Stevens in Benghazi, Libya, the senior Islamic militant called on wealthy Muslims to offer financial rewards to killers of US nationals.

"These prizes have a great effect in instilling fear in the hearts of our cowardly enemies. They also encourage hesitant individuals to carry out important and great deeds in the path of Allah," Gadahn said in a 39-minute video posted on an Islamist website. The speech has been translated by the Washington-based monitoring group SITE.

Al-Qaida in the Arabian Peninsula (AQAP) had earlier announced a hefty reward for the killers of Stevens and three other Americans.

Recently, western diplomatic outposts across the Middle East and North Africa were closed due to a credible al-Qaida threat.

The latest threat is bound to increase tensions in the diplomatic community as most of the closed missions have reopened. Britain has reopened its Yemeni embassy after nearly two weeks.

Gadahn, who grew up on a California goat farm, converted to Islam from his Jewish and Catholic roots.

The US has a $1m (£640,000) bounty on his head and he is widely believed to be holed up in Pakistan.

He rose to prominence following a series of speeches championing al-Qaida, and has previously called for attacks on US missions and diplomats.