A hazard sign at a site hit by an airstrike on Tuesday in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 5, 2017. The hazard sign reads, “Danger, unexploded ammunition”.

Russian Foreign Minister Sergei Lavrov voiced “regret” in a phone call with his US counterpart Friday at Washington’s opposition to a Moscow-backed plan to investigate an alleged chemical attack in Syria.

In the call with Secretary of State Rex Tillerson which “took place at US initiative,” Lavrov raised the probe proposal made by Russia and Iran at the global chemical arms watchdog, the OPCW, said the Russian foreign ministry.

Western nations have accused the Syrian regime of carrying out the suspected April 4 air strike. But Moscow and Tehran, allies of Syrian President Bashar al-Assad, have sought to clear Damascus of blame.

During the Friday call Lavrov notably “expressed regrets about the opposition of the United States at the (OPCW) to an initiative… to send inspectors to Syria to verify reports of the use of sarin gas in the rebel-held town of Khan Sheikhun” which left 87 dead, including many children.

Lavrov and Tillerson “agreed to examine again a possibility of opening an objective probe into this incident under the auspices of the OPCW,” the Organisation for the Prohibition of Chemical Weapons (OPCW), said a ministry statement.

They also agreed to launch “as soon as possible a working group tasked with seeking solutions to tackle points of friction in bilateral relations”, which are at their chilliest since the end of the Cold War.

The US State Department later released a brief statement saying Tillerson and Lavrov spoke to “follow up bilateral issues” and topics “including the OPCW investigation into Syria’s use of chemical weapons on April 4.”

Tillerson “reiterated his support for the OPCW’s existing investigative mechanism,” the statement said, without elaborating.

Ties between Moscow and Washington have been strained by the Ukraine crisis and the Syrian conflict, the Moscow statement said.

The OPCW on Thursday “overwhelmingly” rejected the Russian-Iranian move to launch a new investigation into the suspected chemical attack in Syria, delegates said, backing a probe already under way.

The draft decision put forward by Moscow and Tehran — and obtained by AFP — had called for a new OPCW probe “to establish whether chemical weapons were used in Khan Sheikhun and how they were delivered to the site of the reported incident”.

But it ignored that the body, based in The Hague, is already investigating the attack on the rebel-held town in Idlib province.

The draft had also called for investigators to visit the Shayrat airbase — bombed by the United States after the attack — to “verify allegations concerning the storage of chemical weapons” there.