A huge suicide bomb attack on an Iraqi police base killed at least 37 people Monday, further slowing an operation to retake the city of Ramadi from Islamic State jihadists.

The blast came as US-led coalition members carrying out air raids against IS and providing training and weapons to Iraqi forces prepared to meet in Paris for talks on a string of major battlefield gains by the jihadists in Iraq and Syria.

A suicide bomber blew up his explosives-laden vehicle at a police base in Iraq's Salaheddin province, killing at least 37 people and wounding more than 30, officers said.

"They are mostly policemen," said a doctor at the main hospital in the nearby city of Samarra where the casualties were taken.

Some police officers said the suicide attacker used a tank to muscle his way into the police base, located between Samarra and Tharthar lake, northwest of Baghdad.

The area is being used as part of a military operation aimed at cutting off the Islamic State group's supply lines in Anbar province of western Iraq.

IS fighters have in the past year seized a formidable arsenal of military vehicles, weapons and ammunition from retreating Iraqi forces.

The jihadists' latest haul of gear came on May 17 when Iraqi forces fled and they captured Ramadi, the capital of Anbar province.

The debacle of the security forces prompted Abadi to call in the Hashed al-Shaabi, an umbrella organisation which includes Iran-backed Shiite militias that Baghdad and Washington had been reluctant to involve in the Sunni bastion of Anbar.

- IS gains in Syria -

Iraqi forces launched a counteroffensive but have either stopped on the outskirts of Ramadi or focused efforts on outlying areas in and around Anbar to sever the jihadists' supply lines.

IS used an unprecedented number of massive truck bombs to blast its way into government strongholds in Ramadi and it has since unleashed suicide vehicle-borne bombs on a daily basis.

Abadi vowed after the stinging setback in Ramadi that his troops would wrest it back within days, but he has also admitted the truck bombs were keeping government forces from entering the city.

The premier is headed to Paris for the Tuesday coalition meeting, which will focus on the year-long crisis in Iraq that began when IS overran much of its Sunni Arab heartland last June.

British Foreign Secretary Philip Hammond, on the eve of the meeting, refused to classify the fight against IS as a failure.

"I have no doubt that this will be a generational struggle," Hammond said, giving a time span of around five years.

"This will take a long, long, long time to challenge and overcome the underlying ideology that supports" IS, he said.

"Fragmenting the organisation should be much easier than destroying the underlying ideology."

Coalition strikes have helped Iraqi forces regain ground from IS, but the fall of Ramadi was a stark illustration of the limits of air power in the absence of effective forces on the ground.

The jihadists' capture of the city coincided with their takeover of Palmyra in Syria, in what appeared to swing the momentum in the jihadists' favour after months on the back foot.

On Monday, IS advanced towards Marea, a village between the Syrian city of Aleppo and the Turkish border.

The Syrian Observatory for Human Rights said IS expanded its control in Aleppo province at the weekend, at the expense of rival rebel groups.

The jihadists also gained ground in northeast Syria, where a suicide bomber killed "at least nine regime loyalists" near Hasakeh, the Observatory said.

IS also ousted government forces from areas in the central province of Homs.

"The road is now open (for IS) from Palmyra to Anbar province in Iraq, without any obstacles," said activist Mohammed Hassan al-Homsi.

- Rising death toll -

Geographer and analyst Fabrice Balanche said that across Iraq and Syria, the jihadist group now controlled nearly 300,000 square kilometres (115,000 square miles), an area the size of Italy.

The multiple offensives by IS resulted in a surge of casualties across Syria and Iraq in May.

The Observatory said it had recorded the deaths in Syria of at least 6,657 people last month, the highest number this year.

In Iraq, the health department in Anbar alone reported at least 102 civilian deaths in May.

The United Nations said even partial figures not covering the areas worst affected by the conflict showed at least 665 civilians were killed last month.

Aid agencies are preparing to launch a fundraising appeal for half a billion dollars for the crisis in Iraq, the UN children's agency UNICEF said.

"Five hundred million is really the bare minimum. We're cutting it down to the bare bone," said Philippe Heffinck, its representative in Iraq.

According to the UN, 2.9 million people have been displaced by violence in Iraq since the start of 2014 and almost four times as many have been forced from their homes in Syria since its conflict broke out more than four years ago.

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