Air strikes killed at least 28 civilians in a camp for the displaced in northern Syria near the Turkish border as a 48-hour ceasefire took hold in Aleppo.
The truce came after fierce violence in and around Aleppo and was made possible as President Bashar al-Assad's regime and rebel forces gave in to mounting diplomatic pressure for a pause.
But as relieved civilians went out onto the streets after two weeks of heavy fighting in the divided city, a key battleground in Syria's five-year civil war, others were attacked further west.
The Syrian Observatory for Human Rights said air strikes struck the camp for internally displaced people near Sarmada, in Idlib province, which is controlled by Syria's Al-Qaeda affiliate Al-Nusra Front.
Observatory chief Rami Abdel Rahman said women and children were among 28 civilians killed while 50 others were wounded.
Mamun al-Khatib, director of the Aleppo-based pro-rebel Shahba Press news agency, said "regime aircraft" fired missiles on the camp in the village of Al-Kammouna.
"Two missiles fell near the camp causing people to panic and two more fell inside where a dozen tents caught fire," he said.
Online images showed emergency workers putting out fires among damaged blue and white tents.
The US said it has not confirmed if the strikes were carried out by regime forces, but described them "totally in keeping" with its past operations.
"There's absolutely no justification for attacks on civilians in Syria, but especially on what appears to have been a refugee camp," said State Department spokesman Mark Toner.
The European Union called the bombardment "unacceptable", while the United Nations' top aid official demanded an immediate investigation.
"If this obscene attack is found to be a deliberate targeting of a civilian structure, it could amount to a war crime," said Stephen O'Brien, the UN under-secretary general for humanitarian affairs.
- US, Russia monitoring -
The February 27 ceasefire brokered by the United States and Syrian ally Russia called for an end to fighting between regime forces and rebels nationwide but did not include jihadist-held areas.
Fierce violence in and around Aleppo, which has claimed the lives of more than 280 civilians since April 22, sparked an intense diplomatic push by Washington and Moscow to salvage peace efforts.
Late Wednesday the Syrian army said it had agreed to calls from Russia and the US for a two-day truce in Aleppo that would begin from 1:00 am on Thursday (2200 GMT Wednesday).
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An AFP correspondent in Aleppo said Thursday there had been no signs of fresh air raids.
Residents who had cowered indoors for days emerged and some set up tables and chairs on the streets to enjoy the sunshine, drink tea and smoke cigarettes, the correspondent said.
Shopkeepers also reopened their doors while fruit and vegetable markets -- one of which was struck in an April 24 raid that left 12 dead -- were running again.
The local council dispatched bulldozers to remove rubble in stricken areas where water and electrical supplies were also restored, the AFP correspondent said.
After a whirlwind of talks involving diplomats from top world powers and the UN, US Secretary of State John Kerry announced the truce had taken effect and that violence had already fallen off.
He said US officials in Geneva were coordinating with their Russian colleagues on "enhanced monitoring efforts for this renewed cessation".
The Russian defence ministry said its ceasefire monitors had agreed with their US counterparts to oversee this truce until midnight on May 6.
In Aleppo, the head of the local branch of the powerful Jaish al-Islam (Army of Islam) rebel force, Ahmad Sanada, told AFP the group would respect the truce.
The head of Syria's opposition High Negotiations Committee Riad Hijab meanwhile urged the international community to impose "robust measures" to ensure respect for the ceasefire.
- Palmyra concert -
Diplomats are hoping a nationwide ceasefire can underpin efforts to resolve Syria's five-year war that has already killed more than 270,000 people and forced millions out of their homes.
Mediators hope that UN-backed peace talks could resume later this month in Geneva, although previous rounds have failed to make any major breakthrough with the regime rejecting the opposition's demand that Assad step aside as part of a political transition.
On Thursday, a suicide attack and a car bombing in central Homs province killed at least 12 civilians, the Observatory and state television said.
The twin bombings came amid recent fighting in the area between Islamic State group fighters and regime troops.
IS claimed responsibility for the attacks, according to SITE Intelligence Group.
The area is near Palmyra where on Thursday Russian maestro Valery Gergiev led Saint Petersburg's celebrated Mariinsky orchestra in front of a crowd of Russian soldiers, government ministers and journalists.
Palmyra, a UNESCO world heritage site, was retaken from IS on March 27 with Russian support.