The United Nations said Tuesday that tens of thousands of Syrians have been displaced in the recent fighting between rebels and the government in northwest Syria as the rebels rapidly make territorial gains.

“While thousands have arrived in the central and southern regions of the country, many are trapped at front lines, unable to seek safety in other areas in the country,” Adam Abdelmoula, U.N. resident and humanitarian coordinator in Syria, told reporters in a briefing call from Damascus.

He said intense fighting, shelling and airstrikes have displaced residents from the cities of Idlib, Aleppo and Hama. There have been reports of civilian casualties, but Abdelmoula said the extent is still unclear.

North of Aleppo in Tel Rifaat, an estimated 125,000 people are in the process of moving east, and as many as a third of that number have arrived.

“The latest information received yesterday night, more than 5,000 families, amounting to anywhere between 30,000 to 40,000 individuals, have moved and have entered the northeastern region,” Ramanathan Balakrishnan, U.N. regional humanitarian coordinator for the Syria crisis, told reporters from Amman, Jordan, on the same briefing call.

On November 27, multiple rebel groups aligned under the leadership of Hayat Tahrir al-Sham, or HTS, launched their biggest challenge in years to the regime of President Bashar al-Assad.

They swept into Aleppo, which the government had held since 2016, stunning the Syrian army and capturing the airport, a military academy and much of the city. They also took towns and villages in Idlib governorate, and on Tuesday were approaching the city of Hama.

HTS, which was formerly affiliated with al-Qaida, is a U.N. Security Council-designated terrorist group. It is also listed by the United States, Britain and the European Union as a terrorist group.

“This is the most significant escalation of hostilities witnessed in Syria since 2019,” Abdelmoula said.

UN urges de-escalation

“Now in the space of a week, we have seen the de-escalation lines agreed in 2020 and earlier entirely unravel,” said U.N. Syria envoy Geir Pedersen. “Further shifts in the northeast threaten to unravel other ceasefire arrangements, too, in place since 2019.”

Pedersen told an emergency meeting Tuesday of the U.N. Security Council that the Syrian parties and key international stakeholders need to be “seriously engaged in meaningful and substantive negotiations” to stop the fighting.

“If we do not see de-escalation and a rapid move to a serious political process involving the Syrian parties and the key international players, then I fear we will see a deepening of the crisis,” Pedersen said by video from Geneva. “Syria will be in grave danger of further division, deterioration and destruction.”

Russia entered the war on the side of the Assad regime in 2015 and has been carrying out airstrikes against rebel positions in recent days in support of the Syrian army.

“From the beginning of the operation, approximately 400 terrorists have been liquidated; more than 600 were wounded,” Russia’s U.N. Ambassador Vassily Nebenzia said of their strikes.

Syria’s ambassador demanded that the council condemn what he said is a terrorist attack against his country.

Both the Russian and Syrian envoys accused the United States of supporting terrorist groups in Syria.

“The representative from the Russian Federation is in no position to lecture us on this issue, particularly when it props up regimes that sponsor terrorism around the world,” U.S. Deputy U.N. Ambassador Robert Wood shot back in response to the allegations.