WASHINGTON, March 20 (Reuters) - The U.S. State Department is offering free flights out of Yemen to non-essential diplomats and family members, after three mortars hit a school near the U.S. Embassy in Sanaa.

"The Department of State authorized the voluntary departure for embassy employees and eligible family members after several explosions targeted the embassy compound on March 18," the State Department said on Wednesday in an advisory warning Americans against travel to Yemen.

"The security threat level remains high due to terrorist activities in Yemen," the State Department said.

Thirteen girls and five Yemeni soldiers were wounded in Tuesday's attack near the U.S. mission in the Yemeni capital.

Yemen said "terrorists" were behind the attack.

The ancestral home of Osama bin Laden, Yemen is viewed in the West as a haven for Islamic militants, dozens of whom are jailed in the Arabian Peninsula country for involvement in bombings of Western targets and clashes with authorities.

Yemen, which joined a U.S.-led campaign against militants after the Sept. 11, 2001, attacks on U.S. cities, has also witnessed a number of militant attacks targeting foreign tourists, oil installations and U.S. and French ships.

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