Harrowing eyewitness accounts mounted on Saturday after Syrian forces backed by helicopter gunships killed at least 25 protesters, prompting the US to toughen its stance on President Bashar al-Assad.

As the turmoil neared the three-month mark, the international outcry grew over Assad's use of deadly force against his own people, with protests planned in more than a dozen world cities including Montreal, New York and Paris.

An estimated 3,000 mourners on Saturday filed through the coastal city of Latakia for the funeral of one of at least nine protesters shot dead by security forces the day before, activists said.

The mourners chanted slogans praising the "martyrs," an activist who was present said.

Around the country, 25 people were killed on Friday, including three in the Qabun district of Damascus, after protesters took to the streets after the main weekly Muslim prayers, activists said.

Fridays have become a rallying point in the revolt against Assad's regime, whose backlash on pro-democracy protests that erupted in mid-March has killed more than 1,200 civilians, rights groups say.

The death toll mounted as detailed accounts emerged from some of the thousands of refugees who fled to Turkey from bloodshed in the northwestern town of Jisr al-Shughur.