Gunmen shot dead an American teacher in the Yemeni province of Taiz on Sunday, two defense ministry officials said.
Al Qaeda in the Arabian Peninsula -- the terror network's affiliate in Yemen -- claimed responsibility for the killing.
In a text message sent to Yemeni media outlets, the group said the victim was spreading Christianity to the Yemeni people, calling him one of the biggest missionaries in the country.
Authorities have not said who killed the teacher, whom the head of security in the province, Mohamed Saidi, identified as Joel Sharm.
Saidi said Sharm had been working at a Swedish-run language center in Taiz since 2010.
The U.S. Embassy in the capital Sanaa said it had heard reports of an American death and was investigating. It did not confirm an American had been killed.
On Thursday, tribesmen demanding the release of prisoners kidnapped a female Swiss teacher in Hodeida on the Red Sea coast, officials said.
Imprisoned tribesmen are being held in the Hodeida Central Prison for disturbing travelers and creating roadblocks.
In Sunday's attack, two men on a motorcycle fired eight shots at the teacher who was on his way to work at a Swedish language center in the provincial capital, the defense ministry officials said.
"We are investigating the killing as this is the first of its kind against a western national in Taiz," one of the defense officials said. Neither wanted to be named because they are not authorized to speak to the media.
Yemen has been fighting al Qaeda in the Arabian Peninsula for years, with mixed results.
In September, a CIA-operated drone attack in Yemen killed American cleric Anwar al-Awlaki, the external operations commander and chief recruiter of English-speaking militants for the group.
While the death may reduce AQAP's ability to plan attacks, the group still poses a danger, and "remains the node most likely to attempt transnational attacks," according to the annual U.S. intelligence community's threat assessment, released in January.
Yemen said Sunday that it had killed 14 suspected militants in the south of the country.
The al Qaeda suspects were killed in Jaar town of Abyan when government forces attacked militant hideouts using air raids and tanks on the ground, two security officials in Abyan told CNN.
The Yemeni navy also participated in the attack, bombarding Jaar for three hours Sunday morning.
No soldiers were killed in the attacks, three government officials said.
Two security officials in Abyan told CNN that the attacks were carried out by Yemeni forces targeting weapons caches in the town of Jaar.
"The attacks on al-Qaeda hideouts will not stop against the terrorists in Jaar until they surrender," one official told CNN.
The towns of Dofas, Zinjibar, and al-Kod were also raided by the Yemen air force.
Residents in Jaar told CNN that they were told by the militants to not leave their homes under any circumstances.
"Ansar al-Sharia called out in the morning that anyone leaving their homes will be at risk. The government raids today targeted a number of residential areas," said Salem Juhaif, a resident of Jaar.
Ansar al-Sharia is a separate Islamist militia allied with AQAP.
Yemen has been beset by political turbulence since the beginning of last year. Protests led to the departure of President Ali Abdullah Saleh from office.
His successor, President Abdo Rabu Mansour Hadi, declared Saturday that people killed in peaceful protests against Saleh were "martyrs" -- a religious term elevating their status -- and that their families will receive compensation.
People wounded in the protests will be offered health care at the government's expense, his presidential decree said.