More than 5,000 Syrians have fled their country into Turkey, according to the UNHCR guardian.co.uk

Heavy shelling and gunfire has rocked the Syrian town of Jisr al-Shughour, two days into a military assault that has caused more than 5,000 refugees to flee into neighbouring Turkey.

The continued assault suggests some groups in the town are resisting the armed forces as the regime tries to crush a sustained challenge to President Bashar al-Assad.

The bombardment has reportedly left much of the city in ruins. Farmland to the north has been torched and residents hiding in the mountains say they have been joined by almost all who had remained behind.

Syria's state-run Sana news agency reported "heavy" clashes after army units backed by helicopter gunships and tanks dismantled explosives planted on roads and bridges leading to Jisr al-Shughour. It claims the army is dealing with foreign-backed armed gangs, which it says have laid roadside bombs, dug mass graves and mounted fierce resistance to the military operation.

Gunfire was also heard in other cities across Syria, including Homs, where tanks had moved into restless areas including the neighbourhood of Baba Amr on Wednesday, and the coastal city of Latakia.

Amateur footage of helicopters circling Syrian towns and cities continues to be uploaded to social media sites and there have been numerous reports of them being used to fire at demonstrations.

Turkey says it expects hundreds, if not thousands, more refugees to cross its southern border in the coming days and has vowed to accommodate them all. Syrians fleeing to the four crossing points nearest to Jisr al-Shughour on Sunday seemed more harried than in previous days. Some ran for the border and jostled for position as they waited for Turkish authorities. Hundreds more could be seen streaming down a hillside towards the Turkish village of Guvecci, where soldiers from a nearby base were waiting to receive them.

Humanitarian groups have again demanded access to the Syrian side of the border, fearing a mounting humanitarian crisis in the 12 mile (20km) stretch of hills and valleys leading to Jisr al-Shughour. Refugees who have made it to safety say thousands of people are sleeping rough, too afraid to move on as military helicopters circle.

Italy has urged Syria to allow humanitarian missions.

Germany added its voice to growing calls for a UN resolution condemning Syria. However, the US and Europe said a Libya-style military intervention remained off the table.

Speaking on Sky News, the British foreign secretary, William Hague, made clear there was "no question" of an international military intervention to protect civilians, as has happened in Libya.

Piecing together events in Jisr al-Shughour is becoming increasingly difficult.

Phone lines to the area remain cut and those fleeing are relying on people passing messages to family members along the way.

Some refugees said those who remained in the hills feared losing all contact with relatives if they crossed the border.

Abu Tahar, a wounded relief worker in hospital in the southern Turkish town of Antakya, said his wife and nine-month-old son were among those hiding in rough terrain. He said he had lost touch with them three days ago and could no longer contact anyone he knew by phone.

He said he had personally transported men shot from military helicopters two days before he himself was shot rescuing wounded people from a large garden in the centre of Jisr al-Shughour on 5 June. "Have you seen the damage to a human body from a 14-inch gun?" he asked, nursing two bullet wounds to his back. "It was terrible. A massacre."

Many Syrians are looking for help from Turkey, which the state media is accusing of incitement.

More evidence is steadily emerging of a large-scale military defection in Jisr al-Shughour last weekend, with refugees claiming that many of those who have stayed behind to confront the Syrian army were mutineers.

Several residents who spoke to the Guardian said there had been a small defection by up to 15 soldiers in early June, which in turn had led to government agents being sent to the town to assess the situation, followed by a mass-defection on 5 June that accounted for many of the 120 people reported killed.

"Government forces are questioning orders to fire at unarmed civilians and defecting," said a businessman in Homs with friends and trade partners in the town. "That is why Jisr is posing such a problem to the regime and the operation is taking so long as the defected soldiers will fight back."