RAMADI - Shiite militia groups converged on Ramadi Monday to help Iraqi security forces wrest the city back from Islamic State fighters who seized it in a deadly three-day blitz.

The effective loss of the capital of Iraqs largest province was Baghdads worst military setback since it started clawing back land from the jihadists late last year.

Days after a rare message from IS supremo Abu Bakr al-Baghdadi urging mass mobilisation, the group came close to also seizing the heritage site of Palmyra in Syria, but the army pinned the jihadists back.

Iraqi Prime Minister Haider al-Abadi, the United States and the leadership of the Sunni province of Anbar had been reluctant to deploy Iranian-backed groups in Ramadi.

They favoured developing local forces, but militia leaders said Monday the past few days had proved the government could not afford to do without the Popular Mobilisation Units (Hashed al-Shaabi), an umbrella for militia groups and volunteer fighters.

Hadi al-Ameri, a key figure in Hashed al-Shaabi and the leader of the Badr paramilitary group, argued Anbars leaders should have taken up his offer sooner.

Badrs TV channel Al Ghadeer 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 defence of their own people".

Various militias announced they had units already in Anbar -- including around the cities of Fallujah and in Habbaniyah -- ready to close in on Ramadi and engage the citys new masters.

- Massive reinforcements -

A spokesman for Ketaeb Hezbollah, one of the leading Shiite paramilitary groups in Iraq, said his organisation 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 said, using an Arab acronym for IS.

Ramadi, which lies 100 kilometres (60 miles) west of Baghdad, effectively fell to IS when beleaguered Iraqi security forces pulled out from their last bases on Sunday.

The jihadists used several waves of suicide car bombs to thrust into government-controlled neighbourhoods on Thursday and Friday.

The groups black flag was soon flying above 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.

"Were continuing to monitor reports of tough fighting in Ramadi and the situation remains fluid and contested," Pentagon spokeswoman Maureen Schumann said late Sunday.

Muhannad Haimour, spokesman and adviser to the Anbar governor, also said fighting was ongoing in some pockets of the city. Iraqi military officials said all main security bases had been abandoned.

- Palmyra relief -

Tensions between Tehran and Washington, Baghdads two main foreign partners, also played out during the battle for Tikrit, which the government took back last month.

The fall of Ramadi and IS expansion in Anbar province are also seen as an increased threat on the city of Karbala, home to one of the holiest shrines in Shiite Islam.

In the Syrian half of the "caliphate" Baghdadi proclaimed last year, IS failed to notch up what would have been another high-profile military victory on the ground.

Government forces repelled an IS advance on the ancient oasis town of Palmyra that had sparked widespread concern that yet another UNESCO heritage site could be destroyed.

"ISs attack was foiled," provincial governor Talal Barazi said Sunday after troops routed the jihadists from the northern part of the modern town of Palmyra, which they had seized on Saturday.

UNESCO has urged both sides to spare Palmyra, which it describes as one of the most important cultural centres of the ancient world.

The Syrian Observatory for Human Rights monitoring group said, however, that IS fighters were just a kilometre from the archaeological site and its museum of priceless artefacts.

It said nearly 300 people have been killed in four days of fighting -- 123 soldiers and their allies, 115 IS fighters and 57 civilians.