Nearly 100,000 people have been displaced since early December by fighting in northwestern Syria, where the regime is waging an offensive against jihadists, the United Nations said on Wednesday.

Russian-backed regime forces launched an offensive at the end of 2017 on the edge of Idlib province, the last in the country still fully outside the government's control.

Idlib province is almost entirely controlled by a jihadist outfit known as Hayat Tahrir al-Sham (HTS) consisting mostly of fighters from a former Al-Qaeda affiliate.

The fighting has seen 99,569 civilians flee southern Idlib as well as north and northeastern parts of neighbouring Hama province between December 1 and January 9, said the UN's humanitarian coordination office UNOCHA.

"Heavy bombardment on many communities in southern and southeastern rural Idlib continued unabated, causing casualties and displacement among civilians and destruction of vital infrastructure," UNOCHA said in a statement.

The UN agency said the situation in Idlib was "extremely chaotic" with newly displaced people reportedly scattered across open areas.

"Due to the large numbers of people moving, many are left with no shelter, which could expose them to various risks, especially as the winter temperatures continue to drop," it added.

Ninety-six civilians, including 27 children, have been killed in Syrian or Russian air strikes on Idlib province since the start of the offensive, said the Britain-based Syrian Observatory for Human Rights.

The regime's goal is to retake the southeast of Idlib province in order to secure a route between Damascus and second city Aleppo, both under its control, according to the Observatory.

Its forces are now close to the military air base of Abu Duhur, their next target, the war monitor said.

More than 340,000 people are estimated to have been killed in the Syrian war, which began in 2011 as the regime brutally crushed anti-government protests. Millions have been displaced.

On his first visit to Syria, UN humanitarian chief Mark Lowcock met with Syrian authorities and travelled to the central city of Homs on Wednesday to see the impact of the conflict on civilians for himself.

"In 2017, the United Nations, through our appeal, raised $1.7 billion to help meet the needs of people all over Syria," he told reporters in Homs.