An Iraqi court sentenced former Sunni MP Ahmed al-Alwani to death on Sunday for murder, a verdict that could damage Baghdad's ties with a powerful tribe that is battling jihadists.

"The central criminal court sentenced Ahmed al-Alwani to death... for his killing of two soldiers," judicial spokesman Abdelsattar Bayraqdar told AFP, without saying when the murders took place.

He has a month to appeal the decision, Bayraqdar said.

Alwani is a member of the Albu Alwan tribe, members of which are fighting against the Islamic State (IS) jihadist group in the Ramadi area of Anbar province, a key front in the war against IS, which has seized key parts of Iraq since June.

Sheikh Omar al-Alwani, a leader of the Albu Alwan, said that any decision about Alwani should be put on hold and that the verdict could harm the fight against IS.

"All the Albu Alwan tribe is standing against (IS) on the side of the government," but "half of the Albu Alwan fighters will withdraw if they actually executed Alwani in these circumstances," the sheikh said, adding that even the former MP's guards were fighting against IS.

He said the government should wait until the fighting is over and IS defeated, then "take any decision it considers appropriate."

Illustrating their importance, the US Department of Defence has requested that Congress authorise $18.5 million in arms, ammunition and other equipment for tribes in Anbar and a further $5.5 million in contingency funding.

The gear includes 5,000 assault rifles, 12,000 grenades, 150 heavy machineguns, 50 82mm mortars and other items, according to a document outlining the request.

- Tribal support 'critical' -

"Failure to equip these forces mean a less effective armed opposition to counter the Islamic State," it said.

"Engagement from Sunni tribes is critical to the long-term defeat" of IS, the document said.

The arrest of Alwani, a prominent supporter of a now-collapsed Sunni Arab anti-government protest movement, was one of the factors that sent Anbar province spiralling into chaos.

He was detained during a raid on his house in late December 2013 that killed his brother Ali and five guards, inflaming Sunni Arab anger with the Shiite-led government, which many Sunnis view as having marginalised and unjustly targeted their community.

The defence ministry said at the time that one security forces member was also killed and five were wounded in the raid.

It said Ali was the target of the raid, but that both brothers and the guards opened fire when security forces arrived.

Ahmed had parliamentary immunity, but the constitution permits MPs to be arrested without their immunity being waived in cases of serious crime.

Just days after the raid, security forces demolished the country's main Sunni anti-government protest camp near Anbar capital Ramadi, setting off a series of events that led to the government losing control of parts of that city and all of Fallujah, farther east.

Almost 11 months later, Fallujah is still entirely out of Baghdad's control and is now a stronghold of the IS, while security forces and allied tribesmen are still battling for control of Ramadi.