SANAA, May 20 (Reuters) - A bomb hidden in a cardboard box killed a child in Yemen on Sunday, exploding as school pupils walked past, a government official said.

The official said it was not clear who was behind the attack, on the outskirts of the historic old city of Sanaa, but that an investigation was underway.

Another passer-by was wounded in the blast, he said.

Yemen, the ancestral home of Osama bin Laden, joined the U.S.-led war against terrorism after the Sept. 11, 2001 attacks in the United States. It has been the site of several major attacks by al Qaeda-linked groups in recent years.

The government is battling both al Qaeda-linked groups and, separately, Shi'ite Muslim rebels in the north of the country, but bombings are unusual in the capital.

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