The US on Tuesday accused Syrian President Bashar Al Assad of a “starve or surrender” policy to win the country’s eight-year war.
And France said that the regime's bombing of hospitals and schools in Idlib amounted to war crimes.
The remarks, at the UN Security Council, were the strongest condemnation yet of the Syrian leader's military campaign to win back territory in the country's north-west in recent weeks.
A dire humanitarian situation in Idlib, the last rebel-held Syrian province, where three million people are stranded, is being compared with Aleppo.
In that battle, the Syrian government backed by Russian air power bombed indiscriminately and won Aleppo back in late 2016, with thousands of civilians killed.
Jonathan Cohen, the senior US official at the UN, said Mr Al Assad was presenting civilians with “a bleak choice”, as it had in eastern Aleppo, Ghouta, Homs and other parts of the country.
“Surrender to regime forces or starve,” Mr Cohen said, accusing the regime of falsely claiming that the war was over and warning that use of chemical weapons would not go unanswered.
“This siege tactic is an unmistakable part of the Assad strategy to seek a military solution to the conflict rather than negotiate a political solution through the UN's good offices."
Beyond Idlib, humanitarian aid is not being allowed to enter the Rukban camp in the south-east, which is near the Jordanian border and houses tens of thousands of displaced Syrians.
“Assad's use of starvation as a tactic continues,” Mr Cohen said. “There's no clearer example of the regime's starve and surrender policy than in Rukban.”
Ursula Mueller, UN assistant secretary general for humanitarian affairs, told the council that at least 160 people had been killed in Idlib this month.
And since April 28 there have been 22 air strikes on medical centres, each of which was a breach of the Geneva Conventions, Ms Mueller said.
The Syrian Air Force and Russia control the skies and have justified their bombings by saying they are aiming at Hayat Tahrir Al Sham, a coalition of smaller groups of fighters, the largest of which is Jabhat Al Nusra, a former affiliate of Al Qaeda.
“All of these attacks occurred in north-western Syria,” Ms Mueller said of air strikes on medical centres.
She told all council members that they were bound by international law.
“Sparing schools and hospitals is not optional,” Ms Mueller said.
She urged the council to condemn such incidents rather than delay or avoid a response under the excuse of a continuing but sporadic peace effort.
“Can't this council take any concrete action when attacks on schools and hospitals have become a war tactic that no longer sparks outrage?” Ms Mueller asked.
“Is there nothing to be said or done when barrel bombs are indiscriminately dropped in civilian areas?
"Millions of battered and beleaguered children, women and men cannot wait for another Geneva round [of peace talks] to succeed.”
Since the war broke out in 2011, Russia has used its power of veto as a permanent member of the council to frustrate UN censure of the Syrian regime.
France's permanent representative to the UN, Francois Delattre, said the priority in Syria was to stop Idlib becoming a new Aleppo.
“The lives of three million civilians, a million of them children, are at stake,” Mr Delattre said, referring to Russia's role in recent air attacks.
“There have been nice words from different parties but the offensive continues in Idlib before our very eyes.
"Behind the pretext of a fight against terrorism, this new offensive from the regime and its allies is part of a will to use force to re-conquer the areas that are still outside of its control, as was the case in Aleppo.”
He said France, like the US, was watching closely after accusations that the Assad regime was using chlorine gas.
But Mr Delattre said the attacks on hospitals were not in doubt and were abhorrent.
“The silence of the council on this subject, for reasons of which we are aware, is as deafening as it is intolerable,” he said.
“The attacks against hospitals and healthcare personnel represent war crimes and the perpetrators should answer for their actions.”
Russian deputy foreign minister Sergey Vershinin, taking the UN seat at the council, accused Hayat Tahrir Al Sham of pushing “fake news” about the use of chlorine gas in Latakia province this month, and said action against militants was justified.
