Islamic State militants have seized another town in the western Iraqi province of Anbar less than a week after capturing the provincial capital, a tribal leader said.

Sheikh Rafie al-Fahdawi said the small town of Husseiba fell to IS when police and tribal fighters withdrew after running out of ammunition.

"We have not received any assistance from the government. Our men fought to the last bullet and several of them were killed," he said.

Husseiba is about four miles east of Ramadi, where IS militants routed Iraqi forces last weekend in their most significant advance in nearly a year.

Sheikh al-Fahdawi said that with the fall of Husseiba, the militants have come closer to the strategic Habbaniyah military base, which is still held by government forces.

"The situation is very critical. The militants are about five kilometres from Habbaniyah base, which is now in great danger," he said.

IS militants yesterday captured the Iraqi side of a key border crossing with Syria after Iraqi government forces pulled out. The fall of the al-Walid crossing, also in Anbar, will help the militants shuttle weaponry and reinforcements more easily across the Iraqi-Syrian border.

Meanwhile, the United Nations World Food Programme said it is rushing food assistance into Anbar to help tens of thousands of residents who have fled the fighting in Ramadi.

According to a statement by the WFP, 25,000 people received emergency food assistance yesterday and supplies for an additional 15,000 displaced people are en route to another area near the militant-held city of Fallujah.

The Iraqi government plans to launch a counter-offensive in Anbar involving Iranian-backed Shiite militias, which have played a key role in pushing back IS elsewhere in the country. The presence of the militias could fuel sectarian tensions in the overwhelmingly Sunni province, where anger and mistrust toward the Shiite-led government runs deep.

Iraq's leading Shiite cleric, Grand Ayatollah Ali al-Sistani, called on the Iraqi government to put together "a wise and precise plan" and to rely on military commanders with field expertise to cleanse the country of the IS group.