A top US disarmament official has accused Russia of being on the wrong side of history on chemical weapon use in Syria, after reports of a chlorine attack in the besieged enclave of eastern Ghouta and the leaking of a UN reportinto the supply of related items from North Korea.

The world’s chemical weapons watchdog is said to have opened an investigation into the latest reports of chemical attacks in Ghouta, where doctors said more than a dozen civilians had been treated for symptoms that matched those of exposure to chlorine gas.

Q&A How bad is the situation in eastern Ghouta and is aid getting in? Show In an attempt to convey the desperate and unyielding misery, the United Nations Children’s Fund released a blank statement on 20 February. A footnote said the agency has no words to describe the “children’s suffering and our outrage”. The UN secretary general, António Guterres, did have words: “Hell on earth." An estimated 400,000 civilians, already starved from years of blockade, are trapped amid relentless air strikes. ​Hundreds of people have been killed in the barrage that started on 18 February. Humanitarian groups are pleading for an urgent ceasefire to allow them inside. Aid workers say Syrian helicopters have been dropping barrel bombs - metal drums packed with explosives and shrapnel - on marketplaces and medical centres. Photograph: Mohammed Badra/EPA

The investigation also comes a day after details of a UN report were leaked, which said North Korea had sent Syria more than 40 items used in ballistic and chemical weapons programmes in the five years from 2012 to 2017.

Bashar al-Assad’s government was supposed to have given up its chemical weapons arsenal under a deal brokered in 2013 between the US and Russia, but the agreement does not include chlorine because of its industrial uses.

A senior US disarmament official said on Wednesday that Russia had violated its commitments as guarantor of the destruction of Syria’s chemical weapons stockpile and preventing the Assad government from using them.

Robert Wood, the US permanent representative to the Conference on Disarmament, told reporters in Geneva: “Russia is on the wrong side of history with regard to chemical weapons use in Syria.”

Residents in the rebel-held enclave of eastern Ghouta endured another day of living in basements and bomb shelters after a five-hour lull gave way to renewed violence, four days after the UN security council called for an immediate month-long ceasefire.

Doctors in the enclave said 23 people had been killed in the last 24 hours, adding to a death toll of more than 500 since airstrikes and shelling by Assad’s forces and their Russian allies intensified 11 days ago. One rescue worker was killed while pulling civilians from under the rubble after an apparent strike targeted paramedics and volunteers from a local civil defence group.

Q&A Which outside powers are involved in the Syrian war? Show Over seven years, the civil war has dragged in multiple foreign nations, turning what started as a pro-democracy uprising into a quagmire of overlapping conflicts.

Global powerhouses including Iran, Israel, Russia, Turkey, and the United States have all, to different extents, engaged their militaries in the conflict. At the same time, they have supported warring factions on the ground, including Iran-allied Hezbollah, Kurdish militia, and the US-backed Syrian Democratic Forces alliance. Islamic State and al-Qaida are also present. Photograph: Bulent Kilic/AFP

Ground clashes also erupted on the outskirts of eastern Ghouta as Assad’s troops tested rebel defences after a week and a half of some of the most intense bombardments of the seven-year war.

Vladimir Putin ordered a daily five-hour pause in the fighting on Tuesday, effectively overriding a security council resolution that called for a 30-day ceasefire and the delivery of humanitarian aid. The Russian presidentalso called for the opening of humanitarian corridors. Residents said the pause had led to a brief respite but that airstrikes had returned with force at 2pm.

“It’s like the Russian and Syrian regime pilots are waiting at the ready for 2pm and immediately the bombing and jets and strikes begin,” said a local journalist in Ghouta. “Though today they’re dividing it between the civilians and the frontlines – they’ve introduced more variety.

“People don’t dare to come out of the shelters because it’s not safe. Some of them haven’t had a proper meal in 72 hours.” The siege and bombardment have also taken a grave toll on doctors and paramedics who are struggling to keep up with the influx of wounded.

The scenario in eastern Ghouta, which is home to nearly 400,000 civilians by UN estimates, is reminiscent of previous campaigns by Assad’s troops with Russian support. The government and its allies took back control of the city of Aleppo in 2016 after a crippling six-month siege followed by ground assault and a deal that led to the evacuation of civilians and rebel fighters.

Q&A How does eastern Ghouta compare to Aleppo? Show Syria’s second city, Aleppo, fell into government hands in December 2016 after four years of resistance from rebels, who at one point held large chunks of the ancient metropolis. But the costs to the city and its people were huge, as hundreds of Russian and Syrian air raids pulverised entire neighbourhoods.

The battle for Aleppo was similar to Ghouta in the tactics used by forces loyal to Assad to eject the militants: a persistent and ferocious bombardment with the apparent aim of forcing rebels into a deal to evacuate. The UN's Syria envoy, Staffan de Mistura, has warned that Ghouta risks “becoming a second Aleppo; and we have learned, I hope, lessons from that”. Photograph: Abdulmonam Eassa/AFP

The failure of the UN security council resolution and Russia’s own initiative highlight the international community’s inability to end the violence in Syria. The UN secretary general, António Guterres, has described Ghouta as hell on earth, the high commissioner for human rights has said the campaign against the enclave is a “monstrous annihilation”, but the mounting condemnation has done little to elicit any compromises from Assad and his allies.

Putin said on Wednesday that Russia had managed to evacuate “quite a big group” of civilians from eastern Ghouta, but gave no details.