Image shows Syrian tanks deployed in the eastern Syrian city of Deir ez-Zor earlier in August [AFP/YouTube]

The Syrian army has stormed a northwestern town near Turkey's border a day after authorities said the military had pulled out of the region, activists and local residents say.

The London-based Syrian Observatory for Human Rights said that troops stormed Saraqeb in northwestern Idlib province early on Thursday, detaining at least 100 people.

"Around 14 tanks and armoured vehicles entered Saraqeb this morning, accompanied by 50 buses, pick-ups and security cars. They started firing randomly and storming houses," a resident who fled Saraqeb, 50km southeast of Turkey's Iskenderun province, said.

The Local Co-ordination Committees, activists who help organise and document the protests, said explosions and gunfire were heard after the army stormed the area.

Idlib province has witnessed intense protests against President Bashar al-Assad's regime.

On Wednesday, an AFP journalist saw dozens of soldiers stream out of Ariha in the south of Idlib province.

A Syrian military source said the troops were pulling out of Ariha after succeeding in their mission "to hunt down saboteurs and armed groups at the request of Idlib's residents".

Damascus blames "armed terrorist groups" for fomenting a popular uprising in Syria that has been ruthlessly suppressed by the security forces.

The crackdown has claimed up to 2,000 lives since mid-March according to rights groups.

Further south, in the central province of Homs, columns of tanks entered Qusayr early on Thursday, a rights activist in the town said, reached by telephone.

"Residents fled into the fields and all communications have been cut with the town," the activist said.

On Wednesday, Oscar Fernandez-Taranco, the UN Assistant Secretary-General, briefed the 15-member Security Council behind closed doors about events in Syria in the week since the council called for an "immediate" halt to the violence.

Taranco was quoted as saying there had been no letup in the deaths of protesters while UN officials had met Syrian diplomats to try to get accurate information.

Taranco's briefing had been "depressing and chilling," Philip Parham, Britain's deputy UN ambassador, later told reporters.

But Bashar Jaafari, Syria's ambassador to the United Nations, said the country's sovereignty was "a red line that must not be crossed".

"We know our commitments, our obligations but at the same time we know what are our rights. And our rights do not stem from any political pressure. They stem from our own political will," he said.

The US, which has called for the Security Council to take a tougher stance, imposed sanctions on Wednesday on the commercial bank of Syria, and its Lebanon-based subsidiary, as well as Syriatel, the largest mobile phone operator.