DUBAI, May 10 (Reuters) - An Islamist group claiming links to al Qaeda said on Thursday that Syrian security forces have killed four of its members while they were trying to cross into neighbouring Iraq, and vowed revenge.

Fatah al-Islam group identified two of the four militants killed as "leaders" who used aliases Abu al-Laith al-Shami and Abu Abdul-Rahman al-Shami, and said: "They were trying to cross to support brethren in the Islamic state of Iraq."

"Let the enemies of God, among the Jews and the worshippers of the cross and their guards, expect what would wrench their hearts and ruin their thrones," the group said in a statement posted on the Internet.

"We vow and promise God that the blood of our brothers will not be wasted."

The group said five Syrian security personnel were killed in the clash. It did not say when the clash took place.

The authenticity of the statement could not be verified, but it was posted on a Web site used by al Qaeda and Iraqi insurgent groups.

Fatah al-Islam broke away from the Palestinian pro-Syrian Fatah al-Intifada (uprising) last year. It claims to have links to al Qaeda, which is waging a violent campaign against U.S. forces and those of the Shi'ite-led government in Iraq.

Baghdad and Washington have often accused Damascus, which has opposed the 2003 U.S.-led invasion of Iraq, of not doing enough to block militants from crossing the border that straddles 605 kilometres (375 miles) of desert.

Syria says it was trying to block the flow of militants across its border.

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