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SANAA (Reuters) - A young Nigerian man charged with trying to blow up a U.S. plane on Christmas Day was not deeply religious when he arrived in Yemen in 2004 but evolved into a devoted follower of Islam, a former teacher said.

Umar Farouk Abdulmutallab, 23, stayed in Yemen for a year between 2004 to 2005, before returning for another stay from August 4 to September 21 of last year, Yemeni officials have said.

A teacher at the Sana Institute for the Arabic Language, who taught Abdulmutallab during his time in Yemen, said he was "closer to being secular" when he first arrived in the country.

"But during his visit last year, he was more committed to praying and Islam," the teacher said.

A fellow student who holds U.S. citizenship said Abdulmutallab had not exhibited any violent tendencies and a community leader in the neighbourhood where the institute is located said he had been very friendly to cleaning staff, even offering them chocolates.

"Nobody expected he would harm anyone," the student said.

Abdulmutallab was taken into custody after being overpowered by passengers and crew as the U.S. passenger jet approached Detroit on Christmas Day after taking off from Amsterdam.

Yemeni authorities have tightened entry-visa regulations for applicants looking to join Arabic language institutes, after the December 25 incident, a Yemeni security official said.