Syrian forces backed by helicopters have killed at least 25 people at nationwide protests for democracy today, as the United States said it is stepping up pressure on President Bashar al-Assad to step down.

The latest deaths came as security forces launched a long-feared crackdown in the northwest flashpoint town of Jisr al-Shughur near the border with Turkey.

Protestors poured on to the streets of main towns and cities after the weekly Muslim main prayers, many chanting slogans against Assad and in support of Jisr al-Shughur residents.

Security forces shot dead at least 25 anti-regime Protestors, including 11 in the northwest, rights activists said.

In Maaret al-Numan in Idlib province, security forces fired on a large crowd and killed at least 11 people, activists said.

Rami Abdel Rahman, head of the London-based Syrian Observatory for Human Rights, reported ‘helicopters flying over’ Maaret al-Numan as another activist who spoke on condition of anonymity said ‘helicopters bombarded the town.’

Abdel Rahman said Protestors seized a police station after security forces fled, and helicopters then fired on the building.

State television reported that ‘intense fire by groups of armed terrorists on a security headquarters caused dead and wounded among the police and security forces’ in the town.

Nine people were also killed in the port of Latakia, Abdel Rahman said.

He said another two were killed in the Bosra al-Harir area of southern Daraa province, an epicentre of pro-democracy protests that have shaken Syria since mid-March.

State television said ‘armed men’ fired at security forces in Bosra al-Harir, killing a security force member and a civilian.

Activists said three civilians were also killed in the Qaboun district of Damascus.

State television said the raid on Jisr al-Shughur came ‘at the request of residents.’

‘Army units have started their mission to control Jisr al-Shughur and neighbouring villages and arrest the armed gangs,’ it said.

One witness told AFP that ‘military forces bombarded the villages around Jisr al-Shughur in their advance on the town.’

‘Soldiers torched wheat fields in the village of Al-Ziyara,’ 15 km southeast of Jisr al-Shughur, he said.

Rights activists said most of the town's 50,000 inhabitants had fled - many to neighbouring Turkey - when tanks and troops began converging on the town midweek and that it was now largely deserted.

State television reported that army divisions had entered Jisr al-Shughur and ‘elements of the armed groups have been arrested.’

It blamed ‘armed terrorist gangs’ on Wednesday as it ran images of ‘massacres’ in Jisr al-Shughur which it said had resulted in the deaths of 120 police and troops on Monday.

Opposition activists said the deaths resulted from a mutiny by troops who refused orders to crack down on Protestors.

It was not possible to independently verify the version of events as foreign journalists are banned from reporting in Syria.

Washington and its allies are looking to increase pressure on Assad, a State Department spokesman said.

‘(Assad) has refused to reform, refused even to make any gesture towards reform other than empty rhetoric,’ Mark Toner told reporters.

‘We will continue to look at ways we can up the pressure on him. What's important here is that we make it clear to him that there's growing pressure against his actions,’ he said.

Assad must ‘allow the transition to take place or get out of the way,’ Mr Toner added.

More than 8,000 Protestors also marched through the Kurdish towns of Ras al-Ain, Qamishli and Amuda in the north today, said activist Hassan Berro.

The Syrian Observatory said other protests were held and gunshots heard in Homs, north of Damascus, but cited residents as saying security forces kept away as more than 7,000 converged on Al-Assi Square, in Hama further north, where at least 60 civilians were killed on 3 June.

Protests were also reported in Dael in Daraa province, and thousands gathered in Damascus's Midan district and the Harasta and Barzeh suburbs.

Syria's opposition had called for the renewed protests under the slogan ‘Friday of the Tribes.’

More than 1,200 civilians, including dozens of children, have been killed in the crackdown over the past three months, rights groups say.

Damascus blames the unrest on ‘armed terrorist gangs’ backed by Islamists and foreign agitators.

Assad has come under fire from Turkey's Prime Minister Recep Tayyip Erdogan, a personal friend, in Ankara's harshest reaction yet to the Syrian turmoil, which has forced more than 3,000 people to seek refuge across the border.

‘Unfortunately they do not behave humanely,’ Erdogan said in a television interview carried by Anatolia news agency, calling the treatment of the bodies of women slain by Syrian security forces an ‘atrocity.’

Erdogan said the crackdown was ‘unacceptable’ and would ‘necessarily’ lead the UN Security Council to step in.

Security Council diplomats held new talks today on a proposed European resolution condemning Syria's deadly crackdown but got no closer to a full vote.

Talks were set to be extended through the weekend.

Meanwhile, Assad is refusing to take telephone calls from UN chief Ban Ki-moon, an outspoken critic of the crackdown on opposition protests, officials said today.

Mr Ban tried to call Assad yesterday but was told the Syrian leader was ‘not available’, UN spokesman Martin Nesirky told reporters.

The UN leader has had three stormy telephone calls with Assad since protests, which rights groups say have left at least 1,100 dead, started in March.

Mr Ban said last month that Assad had complained to him: ‘Why do you keep calling me?’

Assad promised in one call to let UN human rights investigators probe events in Syrian protest cities such as Daraa. The team has not been allowed into Syria, however.

Last week, Mr Ban called the events in Syria ‘violent repression’ and said he was ‘deeply troubled by the continued serious violations of human rights, including disturbing reports of the deaths of children under torture, live ammunition and shelling.’