(Adds detail and Hezbollah statement)

BEIRUT, May 14 (Reuters) - Lebanese Shi'ite Muslim militant group Hezbollah said on Saturday its top military commander Mustafa Badreddine was killed by artillery shells fired by insurgents near Syria's Damascus airport.

Hezbollah announced Badreddine's death on Friday and held a military funeral for him on the same day in its stronghold in southern Beirut.

"Investigations have showed that the explosion, which targeted one of our bases near Damascus International Airport, and which led to the martyrdom of commander Mustafa Badreddine, was the result of artillery bombardment carried out by takfiri groups in the area," Hezbollah's statement said.

"Takfiri" is a word used by the group to refer to hard-line, armed, Sunni Muslim Islamist groups.

Hezbollah is fighting in Syria, backing the government of President Bashar al-Assad against a range of Sunni Muslim groups including Islamic State and the al Qaeda-affiliated Nusra Front. Around 1,200 Hezbollah fighters are estimated to have been killed in the Syrian conflict.

"The outcome of the investigation (into Badreddine's death) will increase our determination ... to continue the fight against these criminal gangs and defeat them," Hezbollah said.

The statement did not say when the attack took place or when Badreddine died.

Announcing his death on Friday, Hezbollah quoted Badreddine as having said he would return from Syria victorious or as a martyr.

Badreddine was sentenced to death in Kuwait for his role in bomb attacks there in 1983. He escaped from prison in Kuwait after Iraq, under the leadership of Saddam Hussein, invaded the country in 1990.

His release from jail in Kuwait was one of the demands made by the hijackers of a TWA flight in 1985, and of the hijackers of a Kuwait Airways flight in 1988.

For years, Badreddine masterminded military operations against Israel from Lebanon and overseas and managed to escape capture by Arab and Western governments.

Badreddine was also one of five Hezbollah members indicted by the U.N.-backed Special Tribunal for Lebanon in the 2005 killing of statesman Rafik al-Hariri, one of Lebanon's most prominent Sunni Muslim figures. Hezbollah denied any involvement and said the charges were politically motivated.

(Reporting by Lisa Barrington; Editing by Andrew Heavens) ((lisa.barrington@thomsonreuters.com; +961)(0)(1954456;))

Keywords: MIDEAST CRISIS/HEZBOLLAH BLAST