The Islamic State group took full control of a border crossing between Iraq and Syria Sunday, tightening its grip on the heart of its self-proclaimed caliphate.

The move gave IS control of the two main roads between Syria and Iraq's province of Anbar, as the jihadists pressed their most devastating offensive in months.

The latest success came a week after IS captured the Iraqi city of Ramadi and days after it seized the historic Syrian city of Palmyra, two of the group's most significant military victories in almost a year.

The jihadists seized Al-Walid border post early Sunday when Iraqi government forces pulled back to a nearby crossing with Jordan. IS had taken the Syrian side of the crossing on Thursday.

"There was no military support for the security forces and there weren't enough of them to protect the crossing," Suad Jassem, the head of Anbar's border commission, said.

"Daesh (IS) now controls both sides of both crossings," she said, referring to another crossing between Anbar and Syria further north that the jihadists seized last year.

The surge by a group described as the most violent in modern jihad raised further questions about the efficiency of the US-led coalition's eight-month air campaign.

Coalition warplanes have conducted more than 3,000 strikes in Iraq and Syria since August 2014 and dozens more were carried out in recent days in a bid to contain the rampant jihadists.

There were seven in Anbar alone in a period of 24 hours straddling May 22-23 as Iraqi government and allied forces began to claw back territory from IS east of Ramadi.

- Ramadi fightback -

On Saturday, Iraqi forces retook Husaybah, a rural town in the Euphrates Valley seven kilometres (4.5 miles) east of Ramadi.

The area's most prominent Sunni tribal leader, Sheikh Rafia Abdelkarim al-Fahdawi, deployed his forces, whose knowledge of the terrain is key, alongside fighters from the Hashed al-Shaabi, an umbrella for Shiite militia and volunteers.

A police colonel said the Husaybah operation also involved local and federal police, and the interior ministry's rapid intervention force as well as the army.

Swift action was seen as essential to prevent IS from laying booby traps across Ramadi, which would make any advance in the city more risky and complicated.

Iraqi forces were also battling IS on other fronts, including at the Baiji oil refinery, about 200 kilometres (120 miles) north of Baghdad.

Elite troops have been defending the country's largest refinery for close to a year and Hashed forces have also deployed there in recent weeks to help repel the jihadists' latest assault.

In Diyala province, which the government claimed to have cleared of IS fighters in January, eight bombs went off almost simultaneously early Sunday, security sources said.

Intelligence had been received of a possible wave of bomb attacks and only 14 people were wounded in the blasts in the towns of Baquba and Baladruz, a senior official said.

A top official said he feared more attacks and said Baquba was sealed off as the authorities tightened security around the provincial capital, about 60 kilometres northeast of Baghdad.

- Stranded civilians -

In Syria, analysts said the capture of Palmyra potentially put IS in a position to mount offensives on the capital Damascus and third city Homs.

Palmyra is considered one of the world's archaeological jewels and its capture has raised fears that Syria's most iconic heritage site could be wrecked.

IS militants have smashed statues and blown up ancient artefacts and ruins in several sites in Iraq and Syria.

On Saturday, the head of Syrian antiquities said the jihadists had entered the Palmyra museum but most of the antiquities there had already been transferred to Damascus.

The IS advance in both countries has forced tens of thousands of civilians from their homes, sparking concern among aid agencies.

The fall of Ramadi displaced at least 55,000 people, who join the more than 2.8 million people made homeless by fighting nationwide since the start of 2014.

Aid agencies and some politicians have urged the Shiite-dominated government to allow displaced families who have been held up at a bridge for days to be allowed across to seek shelter in Baghdad and beyond.

The authorities are requesting the displaced to have a sponsor before they can enter the capital, where some accuse them of being infiltrated by IS.