Some 1,000 Syrians who sought refuge in Jordan have been returning home each month since July when a ceasefire for southern Syria took force, the UN said on Monday.

The number of refugees who returned voluntarily to Syria rose to 1,203 in August and 1,078 in September since a ceasefire brokered by Jordan, Russia and the United States for the southern Syrian provinces of Daraa, Quneitra and Suweida on July 9.

"The number of Syrians returning to the country voluntarily has increased", Mohammed al-Hiwari, spokesman for the UN refugee agency (UNHCR) in Amman, told AFP.

"Today that number has risen to around 1,000 [returnee] per month on average," Hiwari added.

In the six months before the cease-fire, a total of only 1,700 Syrian refugees returned to their home country, he said.

Hiwari stressed that the UNHCR "does not encourage the return to zones in Syria that are deemed unsafe."

Jordan shares a border of more than 370 kilometres with Syria, where upwards of 340,000 people have been killed and millions displaced since its conflict broke out in 2011.

The United Nations says Jordan is hosting more than 650,000 Syrian refugees, but authorities in the kingdom put their actual number at 1.3 million.

Hosting the refugees has placed a heavy burden on Jordan, a country lacking natural resources.

The ceasefire brokered in the three southern Syrian provinces is part of a broader Russian-backed plan to create four "de-escalation zones" in rebel-held parts of the country.

In May, Russia and Iran, main allies of the Syrian government, and rebel-backer Turkey agreed to create the four zones in a deal aimed at bringing about a lasting truce.

The latest comments comes as Syria continues to witness a deterioration of the humanitarian situation, with attacks on civilians continuing, Eastern Ghouta held under siege and aid blocked from entry.

The World Health Organisation has also expressed 'grave' concerns over the deteriorating situation in Eastern Ghouta with the needs of up to 400,000 people besieged not met.

The statements come at a time when the United Nations revealed that more than 13 million Syrians are in need of aid.

The Syrian conflict began when the Baath regime, in power since 1963 and led by Assad, responded with military force to peaceful protests demanding democratic reforms during the Arab Spring wave of uprisings, triggering an armed rebellion fuelled by mass defections from the Syrian army.

The brutal tactics pursued mainly by the regime, which have included the use of chemical weapons, sieges, mass executions and torture against civilians have led to war crimes investigations.