The Syrian army says it has temporarily halted combat operations in the south of the country ahead of Russian-sponsored ceasefire talks with rebel groups in Kazakhstan, according to state media.

A statement by the military said hostilities would stop in the provinces of Deraa, Quneitra and Sweida, SANA news agency reported on Monday.

"In order to support the peace process and national reconciliation, a cessation of hostilities ... will last until midnight on July 6," the statement said.

The unilateral freeze was not expected to include fighting against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) armed group.

The Syrian Observatory for Human Rights, a UK-based monitor tracking developments in Syria's war via a wide network of sources on the ground, confirmed to the AFP news agency that fighting "almost completely stopped" across the mentioned areas.

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The news came after a large Syrian rebel faction in the south said it would not attend a new round of talks in the Kazakh capital, Astana, because the government was not abiding by previous ceasefire agreements.

A spokesman for the Southern Front, a coalition of Free Syrian Army rebel groups, cast doubt on whether the Syrian army and its Iranian-backed allies would halt attacks on the front lines in Deraa and in Quneitra province.

"The Free Syrian Army are very distrustful of the regime's intentions in abiding by the ceasefire. It will be like the previous one," Major Issam al-Rayes told the Reuters news agency.

Delegates are expected to begin meeting with a UN mediator and other diplomats on July 4.

The two sides have held four previous rounds of talks in Kazakhstan since January in parallel to UN-brokered peace talks in Geneva. Neither process has made much progress.

A ceasefire declared in May, which is built around so-called "de-escalation zones", has been repeatedly violated.

Khan Sheikhoun attack

Meanwhile, Syrian Deputy Foreign Minister Faisal Mekdad questioned the credibility of the Organization for the Prohibition of Chemical Weapons (OPCW), saying the inspectors had failed to visit key sites linked to a nerve gas attack that killed 89 people in April.

Mekdad dismissed an OPCW report released last week confirming the use of sarin gas on the opposition-held town of Khan Sheikhoun. He said the inspectors refused government invitations to visit the site of the attack in northern Syria and the military airport allegedly linked to it.

The report, which drew on samples taken to Turkey, did not say who was responsible for the attack. The results will be turned over to a joint commission with the UN to apportion blame.

READ MORE: Nearly half a million displaced Syrians return home: UN

Syria insists it has never used chemical weapons.

The Pentagon said last week that intelligence showed the Syrian military preparing another chemical weapons attack at the same airbase the US said was used to launch the Khan Sheikhoun attack on April 4.

US President Donald Trump ordered punitive missile strikes on the Shayrat airbase less than a week after that attack.

Russia, a close ally of Syrian President Bashar Assad's government, called the US statement a provocation.

Syria's war has killed hundreds of thousands of people and forced millions from their homes since it began in March 2011.