Iraq's government marked its first anniversary Wednesday in turmoil as its Sunni vice president disputed charges he ran a hit squad and the US urged calm in a row that has heightened sectarian tensions.

A year after Prime Minister Nuri al-Maliki's unity government was approved by MPs, Vice President Tareq al-Hashemi has holed up in the northern Kurdish region as authorities issued a warrant for his arrest, a Sunni deputy premier faces the sack, and the main Sunni bloc has boycotted cabinet and parliament.

All this comes just days after US troops completed their withdrawal from the country, leaving behind what US President Barack Obama described as a "sovereign, stable, and self-reliant Iraq."

But on Tuesday, Vice President Joe Biden held a telephone call with Maliki, urging him to work with other parties to resolve the worsening crisis that threatens Iraq's fragile political truce.

Biden spoke by phone with Maliki and with parliament speaker Osama al-Nujaifi "to discuss the current political climate in Baghdad," the White House said.

"The vice president told both leaders that the United States is monitoring events in Iraq closely," it added.

Biden also "stressed the urgent need for the prime minister and the leaders of the other major blocs to meet and work through their differences together."

The State Department in Washington, meanwhile, called for Iraq to conduct Hashemi's investigation "in accordance with international legal norms and full respect for Iraqi law."

US officials also confirmed Central Intelligence Agency Director David Petraeus, the former US military commander credited with containing sectarian violence in Iraq, had paid a visit to Baghdad in recent days.

But they said it was a previously scheduled trip to Iraq and Afghanistan and that Petraeus was not engaged in political talks in Baghdad.

Hashemi, meanwhile, held a defiant news conference in the Kurdish regional capital Arbil, denying the charges laid against him and vowing to face them off in court.

"I swear to God that I never committed a sin when it comes to Iraqi blood," he told reporters on Tuesday. "I suggest transferring the case to Kurdistan. On this basis, I will be ready to face trial."

He called for representatives of the Arab League to take part in the investigation and any questioning, and said apparent confessions aired on state television linking him to attacks were "false" and "politicised".

Officials issued the warrant for Hashemi's arrest on Monday, after earlier banning him from travelling overseas.

At least 13 of the vice president's bodyguards have been detained in recent weeks, security officials have said, although it was unclear how many remain in custody.

Hashemi's office said only three were arrested, and has complained of "intentional harassment" in the form of blockade of his home by security forces for several weeks, as well as other incidents.

Maliki and other leaders have called for talks to resolve the political crisis, but the premier's spokesman told AFP he would not accept any mediation over the charges against Hashemi.

"The prime minister will not compromise the blood of Iraqis, no matter what the price," Ali Mussawi said.

Maliki has also called for Deputy Prime Minister Saleh al-Mutlak, like Hashemi a Sunni Arab and a member of the Iraqiya bloc, to be sacked after Mutlak said the premier was "worse than Saddam Hussein".

Lawmakers are due to consider Maliki's request to fire Mutlak on January 3.

Iraqiya said it would boycott the cabinet to protest against Maliki's "dictatorship," although it has not pulled out of the government.

The bloc, which holds 82 of the 325 seats in parliament and controls nine ministerial posts, had earlier said it was suspending its participation in the legislature.

Iraqiya, which garnered most of its support from the Sunni minority and emerged with the most seats in March 2010 elections, was out-manoeuvred for the premiership by Maliki who finished second in the polls but subsequently broadened his power base by striking a deal with another faction.

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