BAGHDAD (Reuters) - The U.S. military said on Sunday it had evidence Iranian-backed Shi’ite militias in Iraq were increasingly using secret weapons stores to attack U.S. and Iraqi forces.

The accusation comes days after Tehran postponed talks with the United States on improving security in Iraq for “technical reasons”, a move that prompted rebukes from U.S. officials.

“In just the past week, Iraqi and coalition forces captured 212 weapons caches across Iraq, two of those inside Baghdad, (which have) growing links to Iranian-backed special groups,” military spokesman Real Admiral Gregory Smith told reporters.

The military uses the term “special groups” to describe rogue elements in the Mehdi Army militia of Shi’ite cleric Moqtada al-Sadr. It says these militants get weapons, funding and training from neighboring Iran.

Smith was speaking at a news conference in which he lauded recent security gains in Iraq, adding that on some days attacks had dropped to below 40 a day, the lowest level since 2004.

Highlighting the fragility of the gains, a female suicide bomber killed at least three people in central Baghdad, police said. The U.S. military said only the bomber was killed.

Two U.S. soldiers were also killed by insurgents in Diyala province northeast of Baghdad, the military said.

Washington, at loggerheads with Shi’ite Iran over its nuclear plans, accuses Tehran of destabilizing Iraq by arming Shi’ite groups. Iran denies the accusations and blames the U.S.-led invasion of Iraq in 2003 for the violence.

Smith said there was no evidence of increased arms shipments to Iraq from Iran, but added that Iranian-backed groups were increasingly using secret stores of weapons to launch attacks.

“What we’re seeing is an increase in the use of weapons by Iranian-backed special groups,” he said, adding the number of weapons caches found in January was the largest in a year.

BITTER ENEMIES

The U.S.-Iranian security talks are one of the few forums in which officials from the two bitter foes have direct contact. Diplomatic ties have been frozen for almost three decades.

Iran’s Foreign Ministry said technical reasons were behind the delay in talks between Iranian and U.S. officials in Baghdad, but did not elaborate. Tehran on Thursday postponed what would have been a fourth round of discussions.

David Satterfield, the U.S. State Department’s Iraq coordinator, said on Friday Iran was “intent on continuing to promote violence within Iraq”.

Violence has fallen 60 percent across Iraq since 30,000 additional U.S. troops became fully deployed in June.

Vital to better security has been a decision by Sunni Arab tribal leaders to turn against Sunni Islamist al Qaeda, and form groups to drive them out.

The U.S.-backed groups, called concerned local citizens (CLCs) by the military, have 80,000 men across Iraq. They man checkpoints and provide intelligence on militant hideouts.

Lt. Gen. Abboud Qanbar speaks to reporters in the heavily-fortified Green Zone in Baghdad October 24, 2007. REUTERS/Maya Alleruzzo/Pool

But cracks have appeared in ties with U.S. and Iraqi forces.

One CLC group said it was suspending its activities after three members were killed in an incident near the town of Jurf al-Sukr, south of Baghdad on Friday.

The unit blamed U.S. soldiers for the deaths. The U.S. military said attack helicopters had responded with rockets after security forces came under small-arms fire.

Smith said any incidents were investigated.

“Coalition forces are working with CLCs to bring about security. They are certainly not targeting CLCs,” he said.

In the southern city of Basra, a CBS News journalist continued to be held by kidnappers after being captured a week ago. Efforts to free the Briton, who has not been named, were being held up over talks about how he should be released, negotiators said.