Anees Mayhoub / AP
The American teacher killed in Taiz, Yemen, on Sunday reportedly was shot while riding in this vehicle.
Updated at 3:25 p.m. ET: An American working in Yemen was shot and killed in the city of Taiz on Sunday, and a group tied to al-Qaida said it had executed the man because of his "Christian proselytizing."
"We can confirm that a U.S. citizen was tragically killed in Yemen," a State Department spokesman told NBC News. "We condemn this heinous crime in the strongest terms and express our deepest condolences to his family and friends."
The U.S. would not name the victim, citing his family's privacy, and conflicting reports surfaced about his identity. Some sources identified him as a Joel Shrun, or Sharm, and others as Joel Wesley.
He was in a vehicle when it was fired on by someone on a motorcycle.
A group linked to al-Qaida claimed responsibility. "This operation comes as a response to the campaign of Christian proselytizing that the West has launched against Muslims," an unidentified person said in a text message to journalists, claiming responsibility on behalf of the Ansar al-Sharia (Partisans of Islamic Law).
Taiz was a hotbed of opposition during earlier protests that forced Ali Abdullah Saleh to step down as president.
Al-Qaida has taken advantage of the security vacuum and is in control of two cities in the south, having staged attacks on the Yemeni police and military, the latest killing more than 150 soldiers.
The attacks underscore the challenges facing President Abd-Rabbu Mansour Hadi who took office last month after a year of massive protests against Saleh.
Taiz, about 120 miles south of the capital Sanaa, is a commercial hub where many foreigners live and work.
Also on Sunday, a government warplane bombed Islamist militants in the southern city of Jaar, causing people to flee their homes, residents said. There were no immediate reports of casualties.
Ansar al-Sharia captured Jaar in Abyan Province in March last year after the outbreak of protests against Saleh and have turned it into their main base in southern Yemen.
Daily clashes break out around areas controlled by militants in southern Yemen. A local official said that up to 14 militants were killed in artillery attacks and clashes on Saturday north of the Abyan provincial capital of Zinjibar, the area of Bagdar and the town of Jaar.
The United States and Saudi Arabia, Yemen's neighbor and the world's biggest oil exporter, are concerned about al-Qaida's expansion in Yemen where it has regrouped after suffering reverses in Iraq and Saudi Arabia.
In early March, militants killed more than 110 soldiers in twin suicide attacks and said they had also captured some 70 soldiers.
Reuters contributed to this report.
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