By Patricia Zengerle

WASHINGTON (Reuters) - The U.S. House of Representatives' special Benghazi committee approved its report on Friday on the attack in Libya that killed four Americans in 2012, but the Republican-led panel set no date for ending its work after a two-year-long investigation.

The committee's chairman, Republican Representative Trey Gowdy, said the panel voted to approve the 800-page report, with seven Republicans in favor and four Democrats opposed, in a closed-door meeting.

Republicans had issued the report on June 28. It included no major new revelations about the assault in Benghazi that killed U.S. Ambassador Christopher Stevens and three other Americans.

But it criticized presumptive Democratic presidential nominee Hillary Clinton, who was secretary of state at the time of the violence, saying both she and her staff showed a "shameful" lack of response to congressional investigators looking into the attack on a U.S. diplomatic compound.

It also accused Clinton's State Department of failing to protect the Americans killed. Last October, Clinton, already a Democratic presidential candidate, calmly deflected harsh Republican criticism of her handling of the attack during a testy 11-hour hearing before Gowdy's committee.

Democrats have derided the report as a political vendetta against the former secretary of state. Republicans said the committee's work would not wrap up until materials related to the report are reviewed for classified information and potential perjury claims, which would be referred to the Federal Bureau of Investigation.

"If you thought a vote today would be the end of this partisan chapter of our history, you would be wrong," Representative Elijah Cummings, the ranking Democrat on the panel, said in a statement.

"The Select Committee is poised to last for months - perhaps until right before the presidential election - and that appears to be the Republican plan," Cummings said.

Republicans plan more interviews, including one next week with Stephen Hedger, the head of legislative affairs at the Department of Defense, Cummings said.

Panel Republicans issued a joint statement calling the investigation "a great privilege and honor." They urged President Barack Obama's administration to clear for public release all of the supporting evidence and documents referenced in the report.

(Reporting by Patricia Zengerle; Editing by Tom Brown)