WASHINGTON — The US government said Friday it will review the decision by a US court to drop charges against American private security guards accused of killing 14 Iraqi civilians in an unprovoked attack in 2007.

"We respect the independence of the judiciary. The decision of the judge does not exonerate the defendants or necessarily terminate the proceedings," State Department spokesman PJ Crowley told reporters.

"The Department of Justice, in consultation with the State Department, will carefully review the judge's decision and assess all available legal options," said Crowley, the assistant secretary of state for public affairs.

The five guards, who had been part of a convoy of armoured vehicles, had been charged with killing 14 unarmed Iraqi civilians and wounding 18 others during the attack at a busy Baghdad square using guns and grenades.

Iraq says 17 people were killed.

But, on December 31, US Federal Judge Ricardo Urbina dismissed the charges against the five guards, saying prosecutors violated their rights by using incriminating statements they had made under immunity during a US State Department probe.

The verdict provoked a furor in Iraq.

"The United States does not tolerate attacks on innocent Iraqi civilians," Crowley said.

"Whatever the case's ultimate outcome, we deeply regret the suffering and loss of life caused by the Nisour Square shootings," he said.

"And we are fully committed to holding accountable persons who commit such crimes, whether at home or abroad," Crowley said.

During a visit to Baghdad, Senator John McCain, the Republican presidential candidate in 2008, said Tuesday he hoped the ruling would be appealed by the US government.

The case was among the most sensational that sought to hold Blackwater employees accountable for what was seen as a culture of lawlessness and lack of accountability in the company's Iraq operations.

Blackwater, which has since renamed itself Xe, pulled out of Iraq in May, after the US State Department refused to renew its contracts.