Shia militias converged on Ramadi Monday in a bid recapture it from jihadists who dealt the Iraqi government a stinging blow by overrunning the city in a deadly three-day blitz.
The loss of the capital of Iraq's largest province was Baghdad's worst military setback since it started clawing back territory from the jihadists late last year.
Days after a rare message from ISIS supremo Abu Bakr al-Baghdadi urging mass mobilization, the group also came close to also seizing one of Syria's most famed heritage sites, ancient Palmyra, but the army pinned it back.
Iraqi Prime Minister Haider al-Abadi had been reluctant to deploy Shia militias to Anbar province for fear of alienating its overwhelmingly Sunni Arab population.
He favored developing locally recruited forces, a policy that had strong US support.
But militia commanders said on Monday that Ramadi's fall had shown that the government could not do without the so-called Popular Mobilization units (Hashed al-Shaabi) – am alliance of Iranian-backed Shia Islamist militias regularly criticized for carrying out war crimes comparable to those of ISIS.
Badr militia chief Hadi al-Ameri said the province's leaders should have taken up his offer of help sooner.
The group's Al Ghadeer television said Ameri "holds the political representatives of Anbar responsible for the fall of Ramadi because they objected to the participation of Hashed al-Shaabi in the defense of their own people".
Various militias announced they had units already in Anbar – including around the cities of Fallujah and Habbaniyah – ready to close in on Ramadi and engage the city's new masters.
A spokesman for Ketaeb Hezbollah, one of the leading Shia Islamist paramilitary groups, said his organization had units ready to join the Ramadi front from three directions.
"Tomorrow, God willing, these reinforcements will continue towards Anbar and Ramadi and the start of operations to cleanse the areas recently captured by Daesh will be announced," Jaafar al-Husseini told AFP, using an Arab acronym for ISIS.
The fall of Ramadi, some 100 kilometers (60 miles) west of Baghdad, came when beleaguered security forces pulled out of their last bases on Sunday.
The jihadists used several waves of suicide car bombs to thrust into government-controlled neighborhoods on Thursday and Friday.
The black flag of the "Islamic State" was soon flying over the provincial headquarters and, with reinforcements slow to come, thousands of families fled the city.
Anbar officials said at least 500 people died in three days of fierce fighting.
"We're continuing to monitor reports of tough fighting in Ramadi and the situation remains fluid and contested," Pentagon spokeswoman Maureen Schumann told AFP late on Sunday.
Muhannad Haimour, spokesman and adviser to the Anbar governor, said fighting was continuing in some pockets of the city. Iraqi military officials said all main security bases had been abandoned.
Tensions between Tehran and Washington, Baghdad's two main foreign partners, also played out during the battle for executed dictator Saddam Hussein's hometown of Tikrit, which the government took back last month.
Abadi met the head of US Central Command, General Lloyd Austin, on Sunday, and on Monday Iranian Defense Minister Hossein Dehghan arrived in Baghdad for talks.
In the Syrian half of the "caliphate" Baghdadi proclaimed last year, ISIS failed to notch up what would have been another high-profile military victory on the ground.
Regime forces repelled an ISIS advance on the ancient oasis town of Palmyra that had sparked concern that another jewel of the Middle East's architectural heritage could be destroyed by the jihadists.
"IS's attack was foiled," provincial governor Talal Barazi said on Sunday after troops ousted the jihadists from the northern part of the modern town, which they had seized on Saturday.
UNESCO has urged both sides to spare Palmyra, which it describes as one of the most important cultural centers of the ancient world.
ISIS fighters remain within a a kilometer (less than a mile) of the archaeological site and its museum of priceless artifacts, the Syrian Observatory for Human Rights said.
The Britain-based monitoring group said nearly 300 people have been killed in four days of fighting – 123 soldiers and militiamen, 115 ISIS fighters and 57 civilians.
AFP contributed to this report.