But while the massacre of members of the Albu Nimr tribe may make it harder for Baghdad to gain more tribal support, tribesmen are still fighting and the killings could yet backfire by encouraging resistance.
The Sunni extremist IS spearheaded a major militant offensive in June that overran much of the country's Sunni Arab heartland, and the government needs the help of influential tribes to regain it.
It is important for Prime Minister Haidar al-Abadi's government to bring the tribes on side not only for military and intelligence support, but also as a show of inclusion to Sunnis, whose anger over marginalisation by the Shiite-dominated authorities has helped IS.
In recent weeks, Abadi has held a series of meetings with tribal leaders in both Iraq and neighbouring Jordan.
"Abadi believes that the role of the tribes is fundamental and key in fighting terrorism," Rafid Juburi, the spokesman for the premier's office, told AFP.
"Communications with the tribal sheikhs are continuing and the prime minister received at least five tribal delegations from Anbar and Mosul and Salaheddin in two weeks," Juburi said, referring to areas where IS holds territory.
'KILL US WITH SILENCE'
In one televised meeting, Abadi told tribal leaders: "We are ready for your requests for weapons and ammunition."
But IS, a ruthless group that has repeatedly carried out atrocities, has struck back at tribal enemies, launching a campaign of executions targeting the Albu Nimr tribe in Anbar province, west of Baghdad.
The tribe has members in both the security forces and anti-jihadist Sahwa militia, and has fought against IS.
IS has executed hundreds of men, women and children from the tribe in recent weeks, a slaughter that Baghdad was unable to stop.
The full death toll remains unclear, but some sources have said more than 250 tribe members have been killed, while others have estimated the toll to be double that figure.
Photos have circulated online apparently showing the aftermath of some of the killings.
One shows a line of more than 30 men in civilian clothes lying in the middle of a street with streams of blood running over the dusty ground, as young men and children look on.
One tribal leader, Sheikh Naim al-Kuoud al-Nimrawi, criticised the government for failing to act.
"We asked... Haidar al-Abadi and his deputy Saleh al-Mutlak to offer help for our tribe, without any result," he told AFP.
Signup to our newsletter and follow us on Facebook and Twitter!
"The homes of the tribes are occupied and their people are occupied -- how did they help us?"
Even Sunni religious leaders were not speaking out against the executions, Nimrawi added, saying they "kill us with their silence".
Lawmaker Ghazi Faisal al-Kuoud, another leader of the tribe, later said that Abadi met with its chief and has agreed to provide weapons and send forces to Anbar to help it.
The killings -- and the government's inability to stop them -- may hurt its attempts to win tribal support.
"This will definitely complicate government and US efforts to bring other tribes on board in the fight against the group," said Kawa Hassan, a visiting scholar at the Carnegie Middle East Centre.
'SURRENDER NOT AN OPTION'
The executions are not only about killing enemies, Hassan said, but "to say to Sunnis the government is weak and cannot protect anti-IS Sunnis, (that) the army is demoralised."
He said the campaign did seem to be deterring Sunnis. "But for how long? This is the big question."
It may in fact have the reverse effect, said Nathaniel Rabkin, the managing editor of the Inside Iraqi Politics newsletter.
"It is of course bad news for the government side to have its supporters massacred," Rabkin said, but the Albu Nimr "appear to still be fighting in other areas of Anbar."
"And there are reports this week that the government has distributed arms to thousands of fighters from the (Albu) Nimr and other tribes," he said.
"If anything, the brutality of the atrocities... may serve to convince other tribes on the pro-government side that when facing (IS), surrender is not an option," Rabkin said.
Sheikh Omar al-Alwani, a leader from the Albu Alwan tribe in Anbar, agreed.
"There were those who believed that the (IS-led) war was taking place to bring victory for the Sunni people, but they are (now) sure that what is happening is a massacre against the Sunni people," Alwani said.
He said that some tribesmen even came back from the relative safety of Iraq's autonomous northern Kurdish region to fight, and that the government has distributed arms.
"All the tribes in Anbar were united against this crime," Alwani said.