A dozen children have been killed in Syria the latest breach of a fragile ceasefire.
Attacks have been reported on schools, hospitals and medical staff and nearly 60 civilians have been killed during escalating violence in the demilitarised region of Idlib over the last fortnight.
The news comes six weeks to the day that Middle East minister Alistair Burt quit in an attempt to avert a no-deal Brexit.
Theresa May has yet to appoint a replacement.
Foreign Secretary Jeremy Hunt condemned the “flagrant violation” of the ceasefire agreement by Russia and the Syrian regime, which has also forced 150,000 people from their homes.
Mr Hunt said it included the use of barrel bombs “for the first time in seven months” and threatened a “swift and appropriate response” if Russia or Turkey used illegal chemical weapons.
He said: “The latest offensive, a flagrant violation of the ceasefire agreement that Russia itself agreed with Turkey, is only compounding what was already a dire humanitarian situation in Idlib.
“Russia and the Asad regime must respect their obligations under the Sochi agreement and international humanitarian law.
“They must also remember that any future use of chemical weapons in Syria would be met with a swift and appropriate response.”
Unicef executive director Henrietta Fore said the ceasefire was broken on April 20 and the violence has continued to escalate.
More than 30,000 people have fled their homes in the last month, with four health facilities destroyed and at least two schools damaged in the same period.
Ms Fore said aid workers could no longer enter the region to provide safe water or medical support.
She said: “The fighting intensified over the past few days, causing our partners on the ground to halt programmes to provide safe water, adequate sanitation and hygiene to affected communities in northern Hama and southern Idlib.
“At least 5,500 people have been left with no water at all.
“Services can only resume once security conditions improve.”
She appealed for children to be protected and called for an end to the fighting so Unicef teams could continue aid work.
“We yet again urge all parties to the conflict and those with influence over them to protect children at all times,” she said.
“Killing and maiming children is a grave violation of children’s rights.
“Civilian infrastructure including health, water and education facilities, are not a target and must not come under attack.”