Syrian activists called protests on Friday over the dozens of children killed in anti-government protests after the opposition demanded President Bashar al-Assad’s “immediate resignation.”

Snubbing government concessions, opposition groups at a meeting in Turkey called late on Thursday for parliamentary and presidential elections within a year of Assad’s ouster and vowed to work “to bring down the regime.”

US secretary of state Hillary Clinton, meanwhile, said the international community needs to be more united on dealing with the Syria Government's crackdown on the pro-democracy movement.

“Right now the attitude of the international community is not as united as we are seeking to make it,” she told reporters in Washington, apparently alluding to Russia’s moves blocking a proposed UN Security Council condemnation of Syria.

Activists called for “Children's Friday” protests to honour the children killed in the uprising, such as 13-year-old Hamza al-Khatib whom activists say was tortured to death, a charge denied by the authorities.

“The people want the fall of the regime. On Saturday, it's ‘Children's Friday’ of rising up against injustice, like the adults,” the activists announced on their Facebook page Syrian Revolution 2011, an engine of the revolt.

The UN children’s agency Unicef says at least 30 children have been shot dead in the revolt against Assad’s autocratic rule which erupted in mid-March.

The more than 10-week-old revolt in Syria was sparked by the arrest and torture of 15 children and adolescents accused of painting anti-regime graffiti in the southern town of Daraa, which became a flashpoint of the deadly protests.

“A photo of a child who is dead or being tortured or being mutilated is much more powerful than of an adult,” said Unicef spokesman Patrick McCormick, referring to the Facebook campaign focused on the fate of Hamza.

“The use of Facebook or any image especially of children is incredibly powerful,” he told AFP. “They are innocent victims here, they get caught in the middle, it is not their fight.”

McCormick warned the situation would worsen with the end of the academic year. “It will leave the children and teenagers more vulnerable because they will be out and about and not sitting in a classroom.”

On the ground, security forces armed with heavy machine guns shot dead 15 civilians in Rastan on Thursday, a human rights activist said, adding to a toll of at least 43 killed in towns of the flashpoint homs region since Sunday.

UN Secretary General Ban Ki-moon’s special advisers on prevention of genocide and responsibility to protect civilian populations expressed alarm on Thursday at the mounting death toll in Syria.

“We are particularly alarmed at the apparently systematic and deliberate attacks by police, military, and other security forces against unarmed civilians,” said advisers Francis Deng and Edward Luck.

More than 1,100 civilians have been killed and at least 10,000 arrested in a brutal crackdown on almost daily anti-regime demonstrations in Syria since March 15, human rights organisations say.

The government insists the unrest is the work of “armed terrorist gangs” backed by Islamists and foreign agitators.