Assailants riding a motorbike on Sunday gunned down an American teacher in Yemen's second city of Taez, an official said, with an Al-Qaeda-linked group claiming it had killed him for preaching Christianity.
The gunmen opened fire on the man while he was in his car in the Sena neighbourhood of the city, 270 kilometres (170 miles) southwest of Sanaa, the security official told AFP on condition of anonymity.
The slain man, whose name has not yet been released, was an American project leader who had worked at the International Training and Development Centre NGO in Taez for two years, sources at the centre said.
Ansar al-Sharia, or Partisans of Sharia (Islamic law) -- a group linked to Al-Qaeda -- claimed responsibility for the attack.
Its fighters "killed an American Christian missionary," said a statement circulated by a mobile phone message and confirmed by a source close to the group.
"The attack is in response to a Western campaign to preach Christianity among Muslims," it said.
The statement confirmed the suspicions of another security official that the attack was the work of Al-Qaeda.
The shooting "carries the fingerprints of Al-Qaeda, but investigations are ongoing" to identify the culprits, said an investigating officer who also asked not to be identified.
The US embassy in Sanaa said it did not have any information about the killing and that it was investigating the report.
Taez's police chief blamed the murder on "saboteurs and outlaws," according to the defence ministry website 26sep.net, and said security forces were hunting down "those behind this cowardly terrorist act to bring them to justice."
The United States says the Yemen-based Al-Qaeda in the Arabian Peninsula, or AQAP, is the most active branch of the global extremist network.
Witnesses said, however, that the attackers were wearing the uniform of the elite Republican Guard, led by the son of former Yemeni president Ali Abdullah Saleh, Ahmed.
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Opponents of Saleh have repeatedly called for the command of the military to be purged of relatives of the veteran leader, who was forced to step down last month after year-long protests inspired by the Arab Spring.
Al-Qaeda's local branch is active in the south and east of Yemen, but not in Taez, which was a major centre for the opposition movement that eventually forced Saleh to step down.
Sunday's attack comes two days after an official said suspected Al-Qaeda gunmen abducted a Swiss woman, also a teacher at a language school, in the Red Sea port of Hodeida and moved her to the restive province of Shabwa farther to the east.
The Swiss foreign ministry confirmed the abduction, saying it had been informed that the woman had been kidnapped late on Wednesday and that it was seeking her release.
Al-Qaeda militants have exploited the weakening central government in Sanaa to strengthen their presence in Yemen, especially across the restive south and southeast.
On Sunday evening, a firefight between government forces and gunmen in the southern port city of Aden left two civilians dead and three wounded, a medical source and witnesses said.
A man and a woman were killed in the clash in the city's central Mualla neighbourhood, with three other civilians wounded, including a woman, a source at the hospital in Aden where the victims were taken told AFP.
Aden has been plagued by violence since Al-Qaeda-linked militants overran several towns in neighbouring Abyan province last May.
Earlier on Sunday in the nearby city of Zinjibar, the provincial capital of Abyan, missiles fired from the sea slammed into Al-Qaeda positions, killing at least 16 suspected militants, a local official said.
The heavy shelling began overnight targeting the northeastern suburbs of Zinjibar, which jihadists have controlled since May following fierce fighting with government troops.
More than 200 people have been abducted in Yemen during the past 15 years, many of them by members of the country's powerful tribes who use them as bargaining chips with the authorities.
Almost all of those kidnapped were later freed unharmed.