The United Nations wants probes to determine whether private security contractors in Iraq have committed war crimes and for governments to ensure that the rule of law is applied, UN officials said on Thursday.

The killing of 17 Iraqis in a shooting involving US security firm Blackwater last month has created tensions between Baghdad and Washington and sparked calls for tighter controls on private contractors, who are immune from prosecution in Iraq.

Ivana Vuco, the UN's senior human rights officer in Iraq, told a news conference that private security contractors were still subject to international humanitarian law and that meant there were specific consequences for any breach.

"Investigations as to whether or not crimes against humanity, war crimes, are being committed and obviously the consequences of that is something that we will be paying attention to and advocating for," she told a news conference.

Iraq says there are more than 180 mainly US and European security companies in the country, with estimates of the number of private contractors ranging from 25,000 to 48,000.

Many Iraqis see security companies as little more than private armies which act with impunity. Iraqi authorities have accused Blackwater of "deliberately killing" the 17 Iraqis in last month's shooting, but the security firm says its guards responded lawfully to a threat against a convoy it was guarding.

This week two women were shot dead when their vehicle ventured too close to an armed convoy. The Australian-owned, Dubai-based security firm Unity Resources Group said the vehicle had ignored warnings to stop and its guards then opened fire.

"Definitely we will keep driving that point home time and again so different groups do not feel above the law in treating the populace," said Said Arikat, UN mission spokesman in Iraq.

In its latest human rights report, covering the period April through June, the United Nations also stressed that the crisis caused by the displacement of Iraqis was getting worse.

The UN estimates there are some 2.2 million Iraqi refugees in neighbouring countries. Inside Iraq, the number of people displaced by violence since the US-led invasion in 2003 is also put at more than 2 million.

"It really is the critical point," said Arikat, who visited a camp in southern Iraq recently. "It is one of the most appalling places for human beings to be anywhere.

"It is shameful and the international community needs to address this issue. It is past urgent," he said.

A rocket or mortar attack on a major US military base in Baghdad on Wednesday night killed two members of the US-led coalition forces and wounded 38 others, the U.S. military said on Thursday.

A Western security contractor at the Camp Victory military base had said he heard nine mortar rounds being fired and four explosions inside the perimeter after dusk on Wednesday.

The number of casualties is the highest in months from an attack on Victory, the US military's sprawling headquarters near Baghdad airport.

Such attacks on the base are relatively rare compared to other US facilities, given its size and location.

In a statement, the military said two "third country nationals", foreign civilian contractors, had also been wounded. It gave no details on the nationalities of any of the victims. Besides US troops, small numbers of soldiers from other countries are based at Camp Victory. - Reuters