The US military said it had evidence Iranian-backed Shi'ite militias in Iraq were increasingly using secret weapons stores to attack US 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 US 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 neighbouring 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 US military said only the bomber was killed. Two US 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 destabilising Iraq by arming Shi'ite groups. Iran denies the accusations and blames the US-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.-Reuters