MANILA, July 11 (Reuters) - A major clash between Philippine marines and Abu Sayyaf militants on a southern island has killed at least 28 people, including eight soldiers, officials said on Wednesday.

Four of the dead marines were found beheaded after the nine-hour gunbattle on Basilan island on Tuesday, marine spokesman Lieutenant Colonel Ariel Caculitan said. About 50 marines and 300 militants were involved in the fighting.

"We were totally outnumbered amd we had no time to recover enemy casualties but we have reports of dead and injured on their side," Caculitan said.

Other officials said at least 20 rebels were killed. At least eight marines were killed, nine wounded and another six were missing, Caculitan said.

It was the single biggest battle in the restive southern Philippines for months. At least 30 rebels, three soldiers and one civilian were killed in clashes on the nearby island of Jolo in April, but that was spread over several days.

The marines on Basilan were checking reports that Giancarlo Bossi, an Italian priest kidnapped on the mainland last month, may have been taken there.

There has been no word on the whereabouts of the priest since he was taken by armed men on June 10 after saying Sunday mass. Caculitan said the marines had not been able to verify his presence on Basilan.

The 300 rebels in the battle were believed to be mainly from the Abu Sayyaf group joined by renegade members of the Moro Islamic Liberation Front (MILF) and some locals, officials said.

The Abu Sayyaf is the smallest but deadliest of several Muslim groups fighting the Philippine government. The MILF is holding peace talks with the government, but many renegades have broken away and are continuing to fight.

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