Damascus (AFP) – International experts preparing to destroy Syria’s chemical weapons arsenal said they had made “encouraging” progress Thursday and expect to carry out on-site inspections within days.

UN Security Council Resolution 2118 was passed after gas attacks outside of Damascus killed hundreds in August, an atrocity that prompted the United States to threaten military strikes on Syria and later led to a rare U.S.-Russian disarmament accord.

The Organisation for the Prohibition of Chemical Weapons and the United Nations said the inspectors, who arrived on Tuesday, had made “encouraging initial progress” following a day of meeting with Syrian authorities.

Nine experts, part of a 19-member team from The Hague-based OPCW, earlier left their Damascus hotel in three cars, heading for an unknown destination.

The team faces a daunting task, as President Bashar al-Assad’s regime is understood to have more than 1,000 tonnes of the nerve agent sarin, mustard gas and other banned weapons stored at dozens of sites.

Their immediate aim is to disable production sites by late October or early November using “expedient methods,” including explosives, sledgehammers and pouring concrete, an OPCW official said.

It is the first mission in the organisation’s history to be undertaken in a country embroiled in a civil war.

The Syrian conflict has killed more than 115,000 people, forced millions more to flee as refugees and trapped hundreds of thousands in besieged towns and neighborhoods.

On Wednesday, the Security Council demanded immediate and “unhindered” access to the trapped civilians, in a non-binding statement that diplomats said sends a strong signal to Damascus.

UN aid agencies say there are more than 2.1 million refugees and nearly another six million people displaced inside Syria. The body has not had access to about two million trapped civilians for months.

The statement says there should be “unhindered humanitarian access” across the conflict lines “and, where appropriate, across borders from neighboring countries.”

Syria has blocked aid missions from those nations, saying supplies will go to rebels.

Since the beginning of the uprising, the council had been deadlocked over Syria, as Russia defended the Assad regime, and last week’s arms resolution and Wednesday statement are a breakthrough.

A top Kremlin official said Thursday that President Vladimir Putin and his U.S. counterpart, Barack Obama, may discuss Syria on the sidelines of a regional summit in Bali next week.

Meanwhile, Human Rights Watch accused Damascus of arbitrarily detaining tens of thousands of people for protesting peacefully, and called for international action to protect them.

Former detainees told the rights group that political prisoners were regularly beaten, raped and shocked with electricity.

The HRW’s Joe Stork called on world powers to pressure the government, as well as opposition groups that have also carried out arbitrary detentions, to free their prisoners.

“Those with leverage with the government as well as with opposition forces should press for them to free everyone they are holding unlawfully,” he said.

Rebels seized the village of Bakar in the southern province of Daraa, as regime forces recaptured a strategic northern town after weeks of battle, the Syrian Observatory for Human Rights said.

The London-based monitoring group said dozens of fighters on both sides died in the battle for Khanasser, which sits on a key supply route between central Syria and the northern city of Aleppo.

Further north, six key rebel factions demanded that an Al-Qaeda front group withdraw from the town of Azaz on the border with Turkey.

The groups, all Islamists, issued a joint call for fighters from the Islamic State of Iraq and the Levant to leave the town, as rebels in central Syria urged the group to leave Homs province.

The appeals came amid fresh clashes between ISIL and the mainstream rebel Northern Storm brigade in Azaz.

Syria’s rebels initially welcomed foreign jihadists but have turned against them in some areas, accusing them of abuses and of imposing on the populace an extreme interpretation of Islam.