WASHINGTON, April 7 (Reuters) - The United States on Monday ordered nonessential embassy staff and family members to leave Yemen a day after an attack on a residential compound in the Yemeni capital.

A Yemeni official said al Qaeda had issued a statement claiming responsibility for an apparent mortar attack on a complex housing Americans and other Westerners in Sanaa, the Yemeni capital.

An al Qaeda affiliate group said earlier in an Internet statement it had fired three mortar shells at the complex.

The U.S. State Department said in a travel warning the security threat level remained high because of terrorist activities in Yemen

"The Department of State ordered the departure from Yemen of nonemergency American employees of the U.S. Embassy and eligible family members following the April 6, 2008 attack on the Hadda residential compound in Sanaa in which three explosive rounds were fired into the compound," the travel warning said.

It also urged U.S. citizens not to travel to Yemen, viewed by the West as a haven for Islamic militants.

"The department remains concerned about possible attacks by extremist individuals or groups against U.S. citizens, facilities, businesses and perceived interests," the State Department said.

Al Qaeda claimed responsibility for a mortar attack in Sanaa last month which missed the U.S. Embassy but wounded 13 girls at a nearby school.

The State Department offered to fly nonessential diplomats and family members out of Yemen after that attack. (Reporting by Joanne Allen; editing by Todd Eastham)

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