Regime troops battled on Friday to take back the rebel bastion of Khaldiyeh in the central Syrian city of Homs, bombarding the district with shells, the Syrian Observatory for Human Rights said.

Khaldiyeh in the north of the restive city has been pounded intermittently since Friday morning "at a rate of five shells a minute," the Britain-based watchdog said.

Elsewhere in the violence-hit country, at least two members of the security forces were killed in an explosion outside a police station in the northwestern province of Idlib, the NGO added.

At least 58 people were killed across Syria on Thursday, where the 15-month crisis may "spiral out of control," UN and Arab League envoy Kofi Annan told the UN Security Council, unless more pressure is put on President Bashar al-Assad.

Anti-regime activists have called for fresh demonstrations on Friday, under the slogan "Revolutionaries and traders, hand in hand until victory," in an attempt to convince the middle classes in Damascus and the northern city of Aleppo to join the uprising that erupted in mid-March 2011.

A convoy of UN monitors trying to reach the central village of Al-Kubeir on Thursday to investigate the slaughter of at least 55 civilians in the small Sunni farming area was shot at, the Observatory said.