Thousands of Syrian Kurds have poured into Iraq over the past few days, to escape deadly clashes between Kurdish fighters and jihadists and seeking a respite from privation.

The UN says more than 15,000 refugees have crossed into Iraq in the latest influx since Thursday, with more expected to follow.

The sudden influx of Syrians across the border stands in marked contrast to the relatively small numbers of refugees taken in by Iraq in recent months compared to other neighbouring countries and has forced the UN refugee agency to scramble aid to the region.

The vast majority of refugees pouring into Iraq's autonomous Kurdish regions in the north are women, children and the elderly.

Several thousand are being housed at the Quru Gusik camp just west of the Kurdish regional capital Arbil, although it is still under construction and lacks many basic services, with others set to be moved to neighbouring Sulaimaniyah province.

But for many, it provides a welcome respite from the fighting ravaging their home districts in a deadly spin-off from the Syrian civil war.

The UN refugee agency UNHCR said on Sunday that the exodus is unlike anything before in Iraq.

"UNHCR is witnessing a major exodus from Syria over the past few days unlike anything we have witnessed entering Iraq previously," Claire Bourgeois, the agency's Iraq representative, said in a statement.

Syrian government forces pulled out of most Kurdish-majority areas of northern and northeastern Syria last year, leaving Kurdish groups to run their own affairs.

But Al-Qaeda loyalists, who have played a significant role in the rebellion against President Bashar al-Assad's regime, see the region as a vital link to fellow jihadists in Iraq and have been locked in deadly fighting with Kurdish militias in recent months.

"There was war and looting and problems," said Abdulkarim Brendar, who trekked with his five children to Iraqi Kurdistan. "We did not find a morsel (of food), so, with our children, we came here."

The plight of civilians like Brendar and his family prompted Iraqi Kurdistan's regional president Massud Barzani to threaten earlier this month to intervene to protect Syrian Kurds, the latest sign of the conflict's growing cross-border impact.

"We fled because there is war, beheadings and killings, and in addition to that there is no work," said Fadhel Abdullah, who crossed into Iraq from the Kurdish-majority Qamishli area of northeastern Syria.

"The economic situation deteriorated and everything became expensive."

The access of Syrian refugees to Iraq has been erratic, with local political tensions and fears of a spillover of the conflict leading Kurdistan regional authorities to shut the border in May.

Some restrictions were eased last month to allow Syrians to join family members already in Iraq, but the number allowed to cross the border had remained relatively low.

All told, more than 1.9 million Syrians have fled their homeland, with most seeking a haven in Lebanon, Jordan and Turkey.

Iraq hosted nearly 155,000 registered Syrian refugees, most of them Kurds, according to the United Nations, before the latest influx.

"The Kurdistan region has received large numbers of those refugees but it should be an international and Iraqi concern," said Dindar Zebari, the deputy chief of the Iraqi Kurdish foreign affairs department.

Zebari said the Kurdish region had allocated an additional $20 million to its budget for Syrian Kurdish refugees, but would require further help from the UN and Iraq's federal government.

For now, mobile medical teams are carrying out basic medical checks on those who have crossed, with a dozen so far referred to hospitals for diarrhoea and vomiting as a result of the heat.

But with the fighting showing no sign of let-up, the number of civilians wanting to cross the border is unlikely to relent.

"There was a shortage of food in the market, and everything became expensive, from bread to gas canisters, and unemployment was spreading," said Ahmed Karim, whose wife held their three-week-old baby in her arms outside a tent in Quru Gusik.

"We decided to save ourselves before we died of hunger."