The latest deaths - one in Baghdad and the other near Hilla to the south of the capital - brought to 65 the number of US soldiers killed since US President George Bush declared major combat over on 1 May.

"The first was a fatality in Baghdad on Thursday," said Sergeant Amy Abbott.

The second attack killed another soldier with the Marine Expeditionary Force near Hilla, some 100 km south of Baghdad.

No details were however given as to when exactly the assault took place.

Meanwhile, US investigators probing the suicide bombing of the UN headquarters in Baghdad are said to be looking into possible complicity of Iraqi security guards with the attackers.

"But the task is not made easier by the conspiracy theories circulating. We will have to separate as best as we can fact from speculation"

Fred Eckhard,                        UN spokesman 

Quoting a senior American official in Baghdad, the New York Times reported the guards at the UN compound had been agents of the Iraqi secret services prior to the Iraq war.

The UN continued to employ them even after the war.

"We believe the UN's security was seriously compromised," the official told the daily.

"We have serious concerns about the placement of the vehicle and the timing of the attack," the official said.

The truck bomb exploded directly under the third-floor office of Sergio Vieira de Mello, the chief UN envoy in Iraq, killing him together with 22 other people.

But UN officials refused to readily buy the conspiracy theory.

Announcing that UN Secretary-General Kofi Annan was sending his security coordinator to Baghdad to investigate the bombing, his spokesman Fred Eckhard said it was too early to jump to any conclusions.

"But the task is not made easier by the conspiracy theories circulating. We will have to separate as best as we can fact from speculation," Eckhard said.