Ongoing hostilities between the Syrian regime and its allies, and opposition forces in northwestern Syria must end immediately, the U.S. said Tuesday.

State Department spokeswoman Morgan Ortagus told reporters that Washington "continues to be alarmed" by airstrikes being carried out by Russia and the Syrian regime in Idlib province.

"Indiscriminate attacks on civilians and public infrastructure such as schools, markets and hospitals is a reckless escalation of the conflict and is unacceptable," she said. "The relentless violence is generating massive civilian displacement in Idlib."

Ortagus estimated that roughly 300,000 people have been forced to flee violence in Idlib, just hours after the UN said the number of persons displaced from the ongoing conflict is around 270,000.

Some 1.5 million people currently reside in Idlib, roughly half of whom have been displaced from other parts of the country.

Turkey and Russia agreed last September to turn Idlib into a de-escalation zone in which acts of aggression would be expressly prohibited.

The Syrian regime and its allies, however, have consistently broken the terms of the ceasefire, launching frequent attacks inside the de-escalation zone.

Earlier Tuesday, Ursula Mueller, UN Secretary General Antonio Guterres' top humanitarian affairs official, lambasted the Security Council for a history of inaction in Syria, questioning what it will do to prevent another catastrophe, this time in Idlib.

"Can't this council take any concrete action when attacks on schools and hospitals have become a war tactic that no longer sparks outrage?

"Is there nothing to be said or done when indiscriminate barrel bombs are dropped in civilian areas?" she said. "Millions of battered and beleaguered children, women and men cannot wait for another Geneva round to succeed. They need protection -- and your action -- now."