Condemnation mounted Friday over deadly air strikes on a camp for displaced people in northern Syria as the regime and its Russian ally denied involvement amid a fragile truce in Aleppo city.
Women and children were reported to be among 28 civilians killed in Thursday's raids near the Turkish border, which also wounded 50.
The strikes in Idlib province, which is controlled by Syria's Al-Qaeda affiliate Al-Nusra Front and rebel allies, came as a 48-hour ceasefire took hold in the battleground city of Aleppo to the east.
That truce was due to end early Saturday (2201 GMT Friday) after giving residents some respite from two weeks of fighting that killed more than 280 civilians, even as clashes raged south of the city.
The Aleppo halt in fighting is part of international efforts to revive a landmark February ceasefire and galvanise peace talks to end a five-year war that has killed more than 270,000 people and displaced millions.
Mamun al-Khatib, director of the Aleppo-based pro-rebel Shahba Press news agency, accused "regime aircraft" of firing missiles at the camp in Al-Kammouna village Thursday -- an accusation Damascus denied.
"There is no truth in the information in some media that the Syrian air force targeted the displaced camp in Idlib province," the official SANA news agency quoted the military as saying.
Russia's military insisted no aircraft flew over the camp on Thursday, suggesting Al-Nusra could have shelled it.
"There were no flights by Russian or any other aircraft," spokesman Igor Konashenkov told Russian news agencies.
- UN's Ban 'outraged' -
"The camp may have been shelled either on purpose or by mistake by multiple rocket launchers which are currently being used very actively in this area by terrorists from Al-Nusra," Konashenkov said.
The February 27 ceasefire between the regime and non-jihadist rebels does not include areas where the Islamic State group and Al-Nusra are present.
UN Secretary General Ban Ki-moon said he was "outraged" by the attack on the camp and said those responsible must face justice.
A video posted online showed emergency workers covering charred victims with blankets and carrying them away.
Ban demanded once again that the UN Security Council refer Syria to the International Criminal Court so that the tribunal based in The Hague can open up investigations of possible war crimes.
The six-nation Gulf Cooperation Council called on the UN to quickly investigate "this heinous crime".
The United States earlier described the raids as "totally in keeping" with the regime's past operations.
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"There's absolutely no justification for attacks on civilians in Syria, but especially on what appears to have been a refugee camp," State Department spokesman Mark Toner said.
UN rights chief Zeid Ra'ad Al Hussein said the camp's tents could clearly be seen from the air so it was "extremely unlikely" to have been an accident.
"It is far more likely they were deliberate and amount to a war crime," he said.
Regime aircraft have previously targeted rebels other than Al-Nusra Front and IS.
- Prison assault -
Russia also launched air raids in support of Damascus in September, and a US-led coalition has conducted air strikes against IS in Syria since 2014.
Qatar's Foreign Minister Sheikh Mohammed bin Abdulrahman al-Thani met Russian President Vladimir Putin in the Black Sea resort of Sochi "to discuss possible ways to save the political process, to save the civilian population", he told reporters.
South of Aleppo city, clashes between regime forces and jihadists and their allies have killed more than 70 on both sides, the Syrian Observatory for Human Rights said on Friday.
Al-Nusra and allied Islamists seized Khan Tuman and surrounding villages in less than 24 hours, according to the Britain-based monitor.
Pro-regime troops had driven the jihadists out of Khan Tuman in December.
In central Syria, regime forces launched an assault against a prison in the central city of Hama aimed at ending a mutiny, the Observatory said.
The mutiny began on Monday after an attempt to transfer inmates to the military-run Saydnaya prison near Damascus.
Human Rights Watch said it had received WhatsApp messages from inmates inside the prison saying that security forces "were attempting to storm their prison block, using tear gas and rubber bullets."
It said the assault "raises major concerns about possible excessive use of force".
As warplanes hit the Idlib camp on Thursday, Syria's regime celebrated its recapture of the ancient city of Palmyra with a concert in its amphitheatre.
A second concert conducted by Valery Gergiev was being staged in the floodlit amphitheatre late Friday.
Before regime troops backed by Russian warplanes retook Palmyra in late March, the theatre was a backdrop for IS executions.