IDLIB, Syria (Reuters) - Syrians escaping violence in Idlib have arrived in villages near the Turkish border in droves, fearing Damascus will launch a full-blown assault on the rebel stronghold.

More than 30,000 people have fled their homes in northwest Syria since the army and allied forces resumed bombardment last week, the U.N. humanitarian affairs office (OCHA) said on Monday. It said a military offensive could trigger the worst humanitarian catastrophe of the 21st century.

“What happened was destruction all over, burning, something you can’t describe,” said Ali al-Mheymid, 50, who escaped the village of Sarjah in Idlib province with his family last week.

“They did not differentiate between civilians and others,” he told Reuters from a village near the Bab al-Hawa crossing with Turkey. He has moved into a tent there with 13 relatives in a makeshift settlement after paying the landowner rent.

Mheymid, who has five children, said people fled in the hundreds, some bringing only blankets and kitchen supplies. “People are hoping that right now, here is safer than there, but God knows what’s coming.”

If government forces advance further toward them, they would have no choice but to move to the Turkish frontier, he added. “There is no escape.”

Damascus and Moscow say they only target armed factions and seek to end the rule of al Qaeda-linked militants over the Idlib region. Idlib makes up a major chunk of Syria’s northwest corner, the last bastion of opposition to President Bashar al-Assad.

The presidents of Turkey, Iran and Russia met in Tehran last week but did not agree a ceasefire that would prevent an expected offensive.

Ankara, which hosts some 3.5 million refugees, has said it could not take any more if an attack on Idlib drove a new surge of people toward its border. President Tayyip Erdogan wrote on Tuesday that an offensive would spark humanitarian and security risks for Turkey, Europe and beyond.

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The Turkish military, which is deployed across an arc of northwest Syria near Idlib with its rebel allies, has also set up 12 observation posts around Idlib this year.

About half of those displaced so far have moved to camps, while others went to informal settlements, stayed with families or rented housing, OCHA spokesman David Swanson said on Monday.

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Mamdouh Abu al-Saoud, who fled with his wife and three daughters, said he worried the fighting would only get worse.

“What will we do? Every time it follows us, we escape a meter to the north and leave it up to God, where will we go?” he said. His family, along with brothers and cousins, arrived in the informal camp near Bab al-Hawa three days ago.

Saoud said years of war had already forced them to flee many times before, first from their hometown in Hama province, then to live as refugees in neighboring Jordan. They later went to southern Syria, only to move again in a withdrawal deal when government forces defeated rebels there around two months ago.

“We had come (to Idlib) without anything,” he said.

An influx of displaced people has roughly doubled the Idlib region’s population to around 2.9 million people in recent years. Civilians and fighters have poured in, shuttled out of towns and cities where Assad’s military crushed rebels with the help of Russia and Iran.

“The first day of the bombing, we said it might dwindle down. The next day, there was more,” said Hussein al-Okab, 43, who came to the informal settlement with his wife and seven kids. “A warplane struck my uncle’s sheep.”

He said entire villages were emptied.