"Gunmen fired on an oil ministry convoy carrying Iranian oil officials in the centre of Baghdad," the Iraqi official said. "No one in the delegation was hurt but two Iraqi guards protecting the convoy were wounded."

In Tehran, the oil ministry's Shana news agency confirmed the attack, saying the country's deputy oil minister was part of the delegation.

"The Iranian delegation's vehicle on Wednesday was fired on, after they finished negotiating with Iraqi oil officials and were on their way to the electricity ministry," the report said.

"All of the Iranian delegation, headed by deputy oil minister Ali Reza Zeighami, are safe and in good health," the news agency reported.

"The attack did not cause any disruption in the delegation's plan and currently they are in another negotiation session," it added.

The attack came two days after a roadside bomb targeted a French embassy car in south Baghdad. Four French security personnel inside the armoured vehicle were unhurt but seven Iraqis were wounded.

Violence in Iraq has plummeted since peaking in 2006 and 2007, but attacks have been on the rise again in recent months.

In other bomb and gun attacks attacks on Wednesday, five people were killed and 36 wounded, 15 of them policemen.

Osama al-Nujaifi, who left for Washington on Tuesday, will bring up the question of the missing billions, which have been under investigation for years, said Baha al-Araji, head of parliament's anti-graft committee.

Last week, US officials acknowledged that $6.6 billion in Iraqi reconstruction funds had disappeared. Iraq says $17 billion is missing, and was stolen by corrupt US institutions.

"Nujaifi is visiting the United States to discuss several issues, including the missing funds," Araji said. "We contacted the US forces in Iraq (about this issue) but we didn't receive an answer," he said, adding that Baghdad had approached the United Nations in Iraq to help trace the money.

The cash was from the proceeds of Iraqi oil sales after the 2003 US-led invasion. It was placed in the Development Fund for Iraq (DFI), but went missing in 2004, when US envoy Paul Bremer's Coalition Provisional Authority (CPA) was governing Iraq and managing the fund.

The integrity committee said it had sent a letter to the UN office in Baghdad, but a spokesperson for the United Nations Assistance Mission in Iraq (UNAMI) said no such document had been received.

It was a 2003 UN Security Council resolution that authorised the transfer of the vanished Iraqi oil money from the US to Iraq.

In the May 11 letter addressed to the UN in Iraq and seen by AFP, the integrity committee accused US institutions working under the CPA of stealing the money.

"The US institutions (occupation forces) working in Iraq committed a financial crime, stealing the money of the Iraqi people that was allocated for the development of Iraq," said the letter. "The sum was $17 billion," it said.

Iraqi government spokesman Ali Dabbagh said in a statement that the issue of the missing billions is being discussed at the highest levels in Baghdad.

"We call on US institutions linked to the issue to guarantee the rights of Iraq and hand over all accounts (from that period) to Iraqi auditors," Dabbagh said.

The US embassy in Baghdad said it was working with the Iraqi government to account for the funds.

"The US and Iraqi governments share a commitment to transparency and accountability with regards to the history of the Development Fund for Iraq," said embassy spokesman David Ranz.

"Our two governments are working together, and with the Special Inspector General for Iraq Reconstruction, to account for all of the funds expended from DFI to benefit the Iraqi people."

"The issue is not about returning the money," said hardline MP Jawad al-Shehaili.

"It's about revealing that the US side did nothing for Iraq. It gave from the right hand and stole from the left," said Shehaili, who is a member of an alliance led by the radical and anti-American Shiite cleric Moqtada al-Sadr.

Earlier this month, Iraqi authorities asked for visiting US congressman Dana Rohrabacher to leave the country after he called for Baghdad to repay part of the money Washington had spent on Iraq since the 2003 invasion.