An American Al-Qaida militant has called for more attacks on Western diplomats in the Arab world, praising the killers of the U.S. ambassador to Libya on September 11 last year, a U.S.-based monitoring group said on Sunday.

Western nations shut embassies across the Middle East and North Africa early this month, after a warning of a possible militant attack. Many have reopened, and Britain said its Yemen embassy would open on Sunday after being closed for 12 days.

Adam Gadahn, a California-born convert to Islam with a $1 million price on his head, appealed to wealthy Muslims to offer militants rewards to kill ambassadors in the region, citing bounty set for killing the U.S. ambassador to Yemen, Washington-based SITE monitoring group said.

"These prizes have a great effect in instilling fear in the hearts of our cowardly enemies," Gadahn said in the 39-minute video recording in Arabic posted on websites used by Islamist militants, according to SITE.

"They also encourage hesitant individuals to carry out important and great deeds in the path of Allah," he said, in an English transcript on SITE.

The Yemen-based branch of Al-Qaida last year offered 3 kg (106 ounces) of gold for the killing of the U.S. ambassador in Sanaa or 5 million rials ($23,350) for an American soldier in the impoverished Arab state.

U.S. ambassador Christopher Stevens and three other Americans were killed in Libya's Benghazi in 2012 when Islamist gunmen attacked the U.S. consulate during a protest by a mob angry over a film they said insulted the Prophet Mohammad.

Gadahn has called for attacks on U.S. diplomats before. In August 2007, he said al Qaida would target diplomats and embassies in retaliation for U.S.-led military action in Iraq and Afghanistan.

The FBI has been trying to question Gadahn - believed to be in Pakistan - since 2004 and the U.S. government has offered up to $1 million in cash for information leading to his arrest.