There was no sign of troop withdrawal from restive areas of Syria Tuesday morning, as stipulated by a UN-brokered ceasefire proposal, activists said.

"The UN plan was supposed to go into effect at 6 am local time. Instead, several Syrian army tanks which are still positioned at the outskirts of central province of Homs, shelled Al-Khalidiyeh and Bab al-Sibaa neighborhoods," activist Omar Homsi said.

"The tanks and troops are still where they are, no changes on the ground so far," he added.

Rami Abdel Rahman, head of the London-based Observatory for Human Rights, stressed that if the peace plan of UN-Arab envoy Kofi Annan does not work, "Syria would plunge into a civil war."

Activists reported Monday that 155 people were killed across Syria. Syrian forces opened fire across two tense borders, killing a TV journalist in Lebanon and at least two people in a refugee camp in Turkey.

Barack Obama's administration expressed outrage at the violence spilling over the frontiers, saying the Syrian government appeared to have little commitment to the peace plan that was negotiated by Annan.