* Al Qaeda-affiliated group claims responsibility
* Victim called Christian "proselytiser"
* Militants killed in attacks by government forces (Adds U.S. comment)
By Mohammed Ghobari
SANAA, March 18 (Reuters) - Gunmen linked to al Qaeda shot dead an American teacher in Yemen on Sunday, accusing him of Christian "proselytising", and officials said government forces had killed a dozen militants in clashes and attacks on their strongholds.
The incidents underscore the challenges facing President Abd-Rabbu Mansour Hadi, who took office last month after a year of massive protests against his predecessor Ali Abdullah Saleh.
A police source in the city of Taiz said a gunman riding on a motorcycle driven by an accomplice shot the U.S. English language teacher, who was also deputy director of a language school, the Swedish Institute.
Officials from the institute in Taiz, Yemen's commercial hub 200 km (120 miles) south of Sanaa, identified the victim as Joel Shrun and said he was born in 1983.
A U.S. State Department official condemned Shrun's killing. "We continue to work to obtain additional information and urge Yemeni authorities to bring to justice all those responsible for this heinous crime," said the official, who did not want to be named.
The gunmen, who escaped after the attack, were believed to be linked to al Qaeda, the police source said. A group affiliated with the militant network claimed responsibility.
"This operation comes as a response to the campaign of Christian proselytising that the West has launched against Muslims," an unidentified person said in a text message to journalists, claiming responsibility on behalf of the al Qaeda-linked Ansar al-Sharia (Partisans of Islamic Law).
The message called Shrun "one of the biggest American proselytisers".
Islamic militants often accuse Western aid groups of covert religious missionary work.
Later on Sunday, unidentified gunmen clashed with security forces in the southern city of Aden. Two passersby, a man and a woman, were killed and several were wounded, residents and medics said.
The United States and Saudi Arabia, Yemen's neighbour and the world's biggest oil exporter, are concerned about al Qaeda's expansion in Yemen where it has regrouped after suffering reverses in Iraq and Saudi Arabia.
Yemen has seen an escalation of al Qaeda violence since Hadi took office in February vowing to fight the Islamist network, which exploited months of protests against Saleh's 33 years in office to seize swathes of territory.
Daily clashes break out around the areas controlled by the militants in the south, and at times with armed supporters of a southern separatist movement which has also stepped up its activities in the past year.
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.
An official said 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.
Also on Sunday, a government warplane bombed Islamist militants in the southern city of Jaar, causing people to flee their homes. There were no immediate reports of casualties.
In early March, militants killed more than 110 soldiers in twin suicide attacks and a raid on their outposts in which the militant group also said it captured some 70 soldiers.
CASUALTIES OF UPRISING HONOURED
Under the power transfer deal, Hadi was elected for a two-year transitional period to restructure the armed forces and oversee the drafting of a new constitution.
On Sunday, state news agency Saba reported that Hadi decreed that civilian victims of the protests against Saleh's rule be honoured as "martyrs" and ordered that their families be treated on par with families of soldiers killed in the line of duty.
Hadi also made several appointments to key government posts.
Ahmed Masoud al-Alwani was made new chairman of the national airline Yemen Airways (Yemenia), replacing Saleh's son-in-law Abdul Khaleq al-Qadi, who was sacked in December after airline workers disrupted operations.
Hadi also named new heads for the police academy, the Civil Registration Department and the country's al-Thawra hospital.
But critics say Saleh's relatives remain in control of the security establishment and armed forces, fanning suspicions Hadi is incapable of standing up to Saleh's allies.
Those suspicions appeared to have been confirmed by last week's transfer of Saleh's nephew Tareq, to the post of commander of the Republican Guard's third brigade, from the command of the Private Presidential Guards. (Additional reporting by Mohammed Mukhashaf in Aden; Writing by Sami Aboudi and Martina Fuchs; Editing by Andrew Roche)