Iraqi Prime Minister Haidar al-Abadi has made it a priority to pay salaries, arm and train fighters from local tribes and communities, and provide legal guarantees for volunteers, envoy Nickolay Mladenov said.
"This strategy is bearing fruit," Mladenov told the 15-member council.
"Communities are beginning to push back."
The massacre by Islamic State fighters of 322 members of the Albu Nimr tribe spurred cooperation with the government in its campaign to defeat the jihadists, he said.
Mladenov called on all militia groups who are not aligned with the jihadists to enter talks with Baghdad on resolving differences and joining the government's anti-Islamist campaign.
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Abadi took office in September as Iraq was in the throes of a fierce offensive by IS fighters who seized large swathes of territory and brought the country to the brink of collapse.
His government last week reached a deal with Iraq's Kurdish region on oil exports that was seen as a boost to national unity.
The self-administered Kurdish region's peshmerga fighters are on the frontlines in the battle against the jihadists.
Under the cover of US-led airstrikes, government forces scored one of their most significant victories last week when they recaptured Baiji.
The town was the largest to be retaken since the jihadist offensive began in June
At least 10,000 civilians have been killed and almost 20,000 injured in Iraq this year, while nearly 1.9 million people have been displaced, Mladenov said.
"These are devastating times for the country," he said.