By Hossein Jaseb

TEHRAN, May 5 (Reuters) - Iran accused the United States on Monday of a "massacre" of the Iraqi people and said further talks with Washington about improving security in its neighbour would be meaningless now.

The Foreign Ministry also voiced support for Iraqi Prime Minister Nuri al-Maliki in cracking down on "illegal" Shi'ite militias, a few days after an Iraqi delegation visited Tehran to urge it to stop backing such armed groups.

Washington accuses Shi'ite Iran of funding, arming and training Iraqi militias to attack U.S. and Iraqi government forces, despite its public commitment to stabilising Iraq.

Tehran blames the violence on the U.S. presence in Iraq.

The two old foes have held three rounds of talks in Baghdad since May last year on ways of reducing violence in Iraq, but a planned fourth meeting has been postponed repeatedly.

"Right now, what we observe in Iraq is a massacre of the Iraqi nation by the occupying forces," Foreign Ministry spokesman Mohammad Ali Hosseini told a news conference.

"Concerning this situation, talks with America will have no results and will be meaningless."

A unnamed Iranian official was last week quoted as saying Tehran saw no need for more talks with the United States on Iraq until what he described as U.S. attacks on Iraqis stopped, but Hosseini's comments were the first such remarks made publicly.

Iraqi government spokesman Ali al-Dabbagh said in Baghdad on Sunday that Maliki had ordered the formation of a committee to compile evidence of Iranian "interference" in Iraq that would then be presented to Tehran.

The U.S. military said last week "very, very significant" amounts of Iranian arms had been found in Basra and Baghdad during an offensive against militiamen loyal to anti-American Shi'ite cleric Moqtada al-Sadr that began in late March.

Iraqi officials have repeatedly said they do not want their country to become a battleground for a proxy war between the United States and Iran, which are also at odds over Tehran's nuclear plans.

Dabbagh said Iranian officials who met the delegation had denied any meddling in Iraq, and Hosseini said Tehran had always supported stability and security in its neighbour.

"What Iran has repeatedly said ... was its support for Mr Maliki's government," he said. "Iran believes that illegal armed groups that committed crimes should be legally confronted."

"Tehran has repeatedly used its capacity to strengthen stability in Iraq among various groups," Hosseini added.

Ties between Iran and Iraq have improved since Sunni Arab strongman Saddam Hussein was ousted in the U.S.-led invasion and a Shi'ite-led government came to power in Baghdad.

Analysts say Tehran wants to keep a friendly government in charge but also wants to ensure rival Iraqi Shi'ite factions look to Iran as a power broker. (Additional reporting by Parisa Hafezi; Writing by Fredrik Dahl; Editing by Sami Aboudi)

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