Syrian army agrees to halt fire to assist sarin probe: Russia

The Syrian military agreed to halt fire around the rebel-held town hit by a suspected chemical attack last month, as the US-backed Syrian Democratic Forces (SDF) crossed into the Islamic State group's bastion of Tabqa in the north of the country.

Russia's defence ministry on Monday said that Syria's military was ready to halt fire around the rebel-held town of Khan Sheikhun, if experts were sent in to conduct a probe.

"The command of the Syrian armed forces has expressed readiness to cease hostilities in Khan Sheikhun if a special mission of experts is sent there to investigate the events of April 4," the defence ministry said in a statement.

Moscow added that Damascus was "ready to declare a complete moratorium on the activities of its troops, aviation and artillery in the area" if investigators were sent in and also that they would be granted access to the Shayrat airbase allegedly used to launch the attack.

Damascus itself, however, has not yet made any official announcement on the matter. 

A suspected chemical attack left 87 dead, including many children, in rebel-held territory in Idlib province on 4 April, with the West accusing Moscow's ally Syrian President Bashar al-Assad of being responsible.

The Kremlin has consistently sought to deflect blame from Damascus over the incident, pointing the finger of blame at rebels and fiercely decrying a punitive missile strike by the United States against the Shayrat airbase.

Russia has also attempted to cast doubt on a probe being carried out by the OPCW global chemical weapons watchdog, criticising the organisation for purported bias and not sending experts to the scene of the alleged attack in war-torn Syria. 

The OPCW last week rejected a move from Assad backers Russia and Iran to launch a new investigation into the incident. 

Moscow's attempts to start a new probe came as OPCW head Ahmet Uzumcu said that "incontrovertible" test results showed sarin gas or a similar substance was used.

As the wrangling goes on over the suspected chemical attack, Damascus has continued to carry out deadly strikes against the rebel-held region.  

Five people were killed and several people injured Monday after air strikes targeted Khan Sheikhun's market, according to the Britain-based Syrian Observatory for Human Rights. 

SDF enters IS-held Tabqa

In northern Syria on Monday, "the SDF entered Tabqa for the first time after besieging it from all sides", according to the Observatory. 

"They seized control of several points in the town's south and were advancing on its western edges," said Observatory head Rami Abdel Rahman.

In an online statement, the SDF said it had captured IS-held positions in west Tabqa, including a roundabout, and part of a southern district.

"There are now clearing operations in the liberated positions," the SDF said.

Tabqa and the nearby major dam of the same name are considered key prizes in a broader offensive for Raqqa, the de facto Syrian capital of IS's self-proclaimed "caliphate", 55km to the east.

IS is under pressure on several fronts, with government forces attacking it elsewhere in Syria and a US-backed offensive targeting its Iraqi stronghold of Mosul.

The SDF launched its campaign for Raqqa in November and has since captured most of the surrounding province.