The UN's secretary general has called for an investigation into deadly air strikes in Syria last week that were allegedly conducted by Russian jets.
António Guterres expressed deep concern about Thursday's attack on the rebel-held town of Zardana, which activists said killed at least 47 civilians.
The dead included a rescuer who was allegedly targeted.
Russia, which backs Syria's government in the civil war, has said the reports have "nothing to do with reality".
The attack on Zardana was one of the deadliest in the mainly rebel-held northern province of Idlib so far this year.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said it killed at least 51 civilians, including nine children and 11 women.
The Syria Civil Defence, whose rescue workers are widely known as the White Helmets, reported that at least 47 people were killed.
The organisation said one of its personnel was killed and four others injured when missiles were fired at rescuers and paramedics helping the wounded and people trapped under the rubble of buildings hit in the initial strike.
Both the White Helmets and the Syrian Observatory said they believed the strikes were carried out by Russian warplanes.
But Russian media cited the country's defence ministry as denying that its aircraft were involved and noting that there had been violent clashes in the Zardana area that day between rebel fighters and jihadists from an al-Qaeda-linked alliance.
In a statement released on Sunday night, Mr Guterres called for "a full investigation into the attacks, especially allegations that there was also a second strike targeting first responders, to establish accountability".
He recalled that Idlib was part of a de-escalation agreement reached last year and called on its guarantors - Russia and Iran, Syrian President Bashar al-Assad's key allies; and Turkey, which backs the rebels - to uphold their commitments.
Idlib, most of which is controlled by Islamist rebel and jihadist factions, is home to 2.3 million people, 60% of them displaced by the war from other areas.
The UN's regional humanitarian co-ordinator for Syria, Panos Moumtzis, fears that an escalation in fighting in Idlib will force residents to try to flee north into Turkey, because "there is no other location to move them to" inside Syria.
"Our worry is that with the Idlib situation, we may not have seen the worst of the crisis in Syria," he told reporters in Geneva on Monday.
More than 920,000 people were displaced across Syria during the first four months of this year - the highest level since the civil war started in 2011. That brought the total number of internally displaced Syrians to 6.2 million, while another 5.6 million are sheltering in neighbouring countries.