An air strike on a Syrian refugee camp that reportedly killed at least 28 people could amount to a war crime, a senior UN official says.
Stephen O'Brien, the UN humanitarian affairs chief, called for an inquiry into the attack on the Kamouna camp in the northern Idlib province.
Reports say the strike on the rebel-held area was by Syrian or Russian planes but this has not been confirmed.
The attack on Thursday (local time) came a day after the extension of a truce was confirmed.
The Syrian military and non-jihadist rebel forces had agreed to a temporary truce around the city of Aleppo, following pressure from the US and Russia.
But the UK-based monitoring group, Syrian Observatory for Human Rights (SOHR), reported an upsurge in fighting between Syrian forces and al-Qaeda-affiliated jihadists close to Aleppo, killing more than 70 fighters from both sides.
The insurgents, including the al-Nusra Front, are reported to have seized the strategically important village of Khan Tuman back from government forces, which had captured it in December.
A nationwide partial cessation of hostilities has been in place since February, but it has come under severe pressure recently, particularly around Aleppo, where some 300 people have died in clashes over the past two weeks.
The UN has warned that if the nationwide cessation fails, it will be "catastrophic" and could send 400,000 more people heading for the border with Turkey.
"The suspicion will fall initially on the Syrian government and we will want to make sure that they, or whoever it is, are fully held to account for this absolutely abominable act," Mr O'Brien said.
"Be in no doubt that all these terrible acts, wherever they happen and whoever perpetrates them, will not be forgotten and the people who perpetrate them will be held to account."
SOHR reported that 28 people had been killed, including women and children, and that 50 more were wounded.
The Local Co-ordination Committees, an opposition activist network, said about 30 people had been killed and dozens wounded.
Images on its Facebook page showed the aftermath of fires among the blue tents in the camp, with the ground still smouldering.
The Syria Civil Defence, a group of volunteer rescue workers, gave a similar number of civilian deaths.
The White House condemned the attack.
Spokesman Josh Earnest said: "These individuals are in the most desperate situation imaginable, and there is no justification for carrying out military action targeting them."
UK Foreign Secretary Philip Hammond said the attack was "horrifying", adding: "The [Bashar al-] Assad regime's contempt for efforts to restore the cessation of hostilities in Syria is clear for all to see."
Thursday's attack came a day after a partial truce was extended to include Aleppo.
The fighting in Aleppo early this week has been the most intense there for more than a year.
The Observatory said rebels had advanced into government-held western districts on Tuesday night (local time) but were pushed back by Wednesday morning.
Rebels began observing a ceasefire there on Wednesday and the Syrian military announced a 48-hour truce in the city from Thursday morning.
Some rocket attacks were reported on government-held areas on Thursday, but the intense air force strikes of recent days were absent.
The nationwide partial truce agreement does not include the Islamic State (IS) or the al-Qaeda-affiliated Nusra Front.
Fighting involving government forces and jihadist groups continued on Thursday, with clashes reported near Khan Touman, south of Aleppo.
IS militants captured the Shaer gas field in the east of the country, killing 30 troops, monitors said, while Russian jets were reported to have struck militants in Sukhna, north-west of Palmyra in central Syria.
EU humanitarian aid commissioner Christos Stylianides tweeted: "Shocking bombardment of refugee camp on Syria/Turkey border is unacceptable. My thoughts are with the people who suffered already enough."
Shocking bombardment of refugee camp on Syria/Turkey border is unacceptable. My thoughts are with the people who suffered already enough.— Christos Stylianides (@StylianidesEU) May 5, 2016