Sana'a: The United Nations has closed some of its Yemen offices, erected blast walls around its main headquarters in the country and pulled out some nonessential staffers because of security concerns, UN and Yemeni officials said on Sunday.

The changes followed an attack on a housing complex for Western diplomats in Yemen's capital, Sana'a, earlier this month and a mortar attack last month on the US Embassy.

Three mortars missed the American Embassy on March 20 but crashed into a girls high school nearby instead, killing a security guard. Three projectiles also hit a foreigners' compound on April 6, shattering windows but causing no injuries. The complex is in an upscale neighbourhood that also houses UN buildings.

Some UN offices have shut down in recent days, but others, including the world body's refugee agency, the UN High Commission for Refugees, remained open, UNHCR regional spokeswoman Abeer Etefa said by telephone from Cairo, Egypt.

A number of UN staffers at various agencies have already left Yemen, Etefa said.

UN officials in Yemen were not immediately available for comment. Three-metre high blast walls and piles of sandbags have been erected around the United Nations' headquarters in Sana'a. Yemeni security troops were deployed around the building.

No employees have been seen entering or exiting the compound in several days, said a Yemeni security official, speaking on condition of anonymity because he was not authorised to describe the UN measures.

Yemeni security troops already protect foreign missions, hotels and UN buildings, and continue to be able to protect their occupants, he said.