The United Nations expressed deep worry on Wednesday over the fate of more than 40,000 Syrians who have fled fighting near the northern city of Aleppo that has escalated in recent days as government forces press an offensive despite a truce.
The renewed violence included airstrikes in Idlib province which reportedly killed about 40 people in a crowded vegetable market.
The airstrikes and fighting appear to leave a fragile six-week-old ceasefire in tatters. The truce was brokered by the United States and Russia to pave way for the first peace talks attended by the warring factions since fighting began five years ago.
The main Syrian opposition group HNC said Tuesday that the airstrikes marked a "dangerous escalation" one day after the group declared the truce finished and indefinitely suspended peace talks with the Syrian government.
A monitoring group said it believed the strike on the market in Maarat al-Numan in Idlib province was the deadliest single attack since the truce took effect on Feb. 27.
A rescue worker said warplanes had simultaneously struck markets in two towns in Idlib killing at least 38 people in Maarat al-Numan and 10 others in nearby Kafr Nubl.
"We have more than 20 cars that have been moving dead and injured to hospitals in the area," said Ahmad Sheikho, a member of the civil defense corps, a rescue service operating in opposition-held territory.
It was not immediately clear whose aircraft were responsible for the air strikes. Both Syria's own air force and that of its Russian allies have been operating despite the truce.
Damascus and Moscow say they are striking only territory held by Islamist fighters who are not covered by the ceasefire, but opposition groups dismiss this, saying the government and Russia use the Islamists to justify wider attacks.
The British-based Syrian Observatory for Human Rights, which monitors the conflict through a network of sources on the ground, said 37 people were killed in Maarat al-Numan and seven in Kafr Nubl. Many of the injured were in a serious condition and the death toll was expected to rise, it said.
Asked if it was the deadliest attack recorded since the ceasefire began, the Observatory's head, Rami Abdulrahman, said: "I believe so."
Ahmed al-Seoud, head of the 13th Division, a foreign-backed faction fighting under the banner of the Free Syrian Army, also gave a death toll of about 40 in Maarat al-Numan and said 80 people were wounded.
The Syrian military could not immediately be reached for comment, and state news agency SANA made no mention of air strikes in Idlib.
Fighting in neighboring Aleppo province, mainly around the Aleppo city, has become the biggest threat to the fragile cessation of hostilities, and has contributed to the decision by the main opposition delegation to suspend its formal participation in peace talks.
The opposition accuses the government of violating the cessation of hostilities agreement to launch a new offensive to capture Aleppo, Syria's most populous city before the war, which has been divided between government-controlled and rebel-held zones for years.
The government and its Russian allies say they are fighting only against Islamist militants who are not covered by the truce, and blame the opposition for violating the ceasefire in other parts of Syria.
More than 40,000 people in camps, residential areas and settlements have been displaced due to fighting in recent days, mostly pushed eastwards towards the strategically vital border town of Azaz, as well as the Bab al-Salam and Sijjou camps for internally-displaced, the UN Office for Humanitarian Affairs (OCHA) said.
"Taking into account the previous influx of over 75,000 internally displaced people into the Azaz sub-district in January and February, humanitarian needs are expected to rise exponentially," it said.
Previous rebel losses in the area near the Turkish border have made it difficult for international aid agencies to reach civilians, making it one of the areas of greatest concern for those trying to protect Syria's civilians from harm.
"We are extremely concerned at the intensification of fighting in northern Syria and its impact on civilians, as well as humanitarian delivery to the area, and continue to monitor the situation closely," said Ariane Rummery of the UN refugee agency.
Aid agencies have distributed food baskets and blankets, jerry cans, mattresses and plastic sheeting for thousands of newly displaced people and are preparing to scale up the response, she said.
The medical charity Medecins Sans Frontiers said there were now more than 100,000 people trapped on the Syrian side of the Turkish border, with 35,000 having fled in the past week from camps that had been taken over by Islamic State fighters or had become too close to the front line.
Turkey has closed the frontier to all but the most seriously ill or wounded people.
"Yet again we see tens of thousands of people forced to flee but with almost nowhere safe to go — trapped in this bloody, brutal conflict," said Muskilda Zancada, MSF's mission chief in Syria in a statement.
Riad Hijab, chief coordinator of the High Negotiations Committee (HNC), told a news conference in Geneva on Tuesday: "The regime and its allies are working on seizing Aleppo, which means seizing 650,000 citizens. The regime and Russians are using all kinds of weapons."
Peace Talks Collapse
The peace talks taking place under UN auspices in Geneva were already close to collapse after the opposition called a pause on Monday.
As fighting raged and air strikes on rebel-held areas intensified, the opposition urged foreign states to supply them with the means to defend themselves, a reference to the anti-aircraft weapons long sought by insurgents.
US President Barack Obama told Russia's Vladimir Putin in a phone call on Monday that Syria was starting to fray more rapidly. The two Cold War era super-powers are joint sponsors of the peace process and ceasefire, after intensifying diplomacy following Russia's decision last year to join the war, an intervention that tipped momentum Assad's way.
The Geneva talks aim to end a war that has killed more than 250,000 people, created the world's worst refugee crisis, allowed for the rise of the Islamic State group and drawn in regional and world powers.
Damascus blames the rebels for breaking the cessation of hostilities. Chief Syrian government negotiator Bashar Jaafari said his team was pushing for an expanded government as a solution to the war, an idea rejected by the armed opposition which has fought for five years to oust Assad.
With fighting reported across much of northwest Syria on Tuesday, both sides were obdurate. The government says it is willing to negotiate on a national unity government that could include some opposition figures, but not on the question of Assad leaving power.
"Our mandate in Geneva stops at forming a national unity government," Jaafari told Reuters. "We have no mandate whatsoever either to address the constitutional issue meaning establishing a new constitution or addressing parliamentary elections or addressing the fate of the presidency."