Iran-backed Shia militias have been sent by the Iraqi government to Ramadi to recapture the city seized by Islamic State (ISIS) militants on May 17.
About 500 people are reported to have died when the Iraqi military abandoned positions in Ramadi – only 70 miles West of Baghdad.
A regional government official spoke of people fleeing Ramadi, the capital of Anbar province, “in great numbers”.
The US has said it is confident the capture of Ramadi can be reversed.
Speaking in South Korea, Secretary of State John Kerry said: “I am convinced that as the forces are redeployed and as the days flow in the weeks ahead that’s going to change.”
The Shia militias, known as the Popular Mobilization (Hashid Shaabi), were key to the recapture from ISIS of another city, Tikrit, north of Baghdad, in April. But their use has raised concern in the US and elsewhere.
The militias pulled out of Tikrit following reports of widespread violence and looting.
Meanwhile, the Iranian Defense Minister, Hossein Dehghan, has arrived in Baghdad on a visit arranged before the latest developments in Ramadi.
The police and military made a chaotic retreat from Ramadi, which has been contested for months, after days of intense fighting.
A statement purportedly from ISIS said its fighters had “purged the entire city”. It said ISIS had taken the 8th Brigade army base, along with tanks and missile launchers left behind by troops.
ISIS militants have killed at least 50 members of an Iraqi tribe in western Anbar province, officials and tribal leaders say.
The men and women from the Al Bu Nimr tribe are reported to have been lined up and shot in retaliation for resisting the jihadists.
A number of people from the same tribe were also found dead in mass graves earlier this week.
ISIS militants control large areas of Iraq and neighboring Syria.
Meanwhile, AFP news agency quoted the UK-based Syrian Observatory for Human Rights as saying that at least 100 ISIS fighters had been killed in three days of fighting for the strategic Syrian border town of Kobane.
On October 31, some 150 Iraqi Kurdish Peshmerga fighters crossed from Turkey to join Syrian Kurds who have been defending the town against ISIS for six weeks.
The Observatory says that more than 950 people have died in the battle, more than half of them from IS.
A local official told the Associated Press news agency that the Sunni Muslim tribesmen and women were killed on October 31 in the village of Ras al-Maa, north of the provincial capital Ramadi.
Faleh al-Issawi said many members of the tribe had to flee their homes near the town of Hit last month when it was captured by ISIS.
ISIS militants control large areas of Iraq and neighboring Syria
The Al Bu Nimr tribe had joined the Shia-dominated government’s campaign against ISIS.
There have been many other such killings, as pressure mounts on the tribes to swing one way or the other.
Analysts say mass killings are also a very deliberate strategy by ISIS to spread terror in their opponents.
One local official, Sabah Karhout, described the killings in Anbar province as a crime against humanity and called for more international support for Sunni tribes fighting the militants in Anbar.
Defense Secretary Chuck Hagel said the killing of Sunni tribesmen in Iraq by Islamic State fighters was the brutal “reality of what we’re dealing with” in the conflict.
The US carried out an air drop of food supplies, the first of its kind, to the Al Bu Nimr tribe just a few days ago.
ISIS has taken over large parts of Anbar province as it expands its territory, currently about one-third of both Iraq and Syria.