Syria's main opposition group says 1,000 people have been killed by government forces in the last eight days.
A spokeswoman for the Syrian National Council says troops loyal to President Bashar Assad have intensified their onslaught in opposition areas despite saying it would accept a UN peace plan.
SNC spokeswoman Bassma Kodmani says 160 people were killed in Syria on Monday alone.
Kodmani told reporters in Geneva on Tuesday that regime forces have used heavy weapons including anti-aircraft guns against civilians in apparent defiance of an agreement to begin a cease-fire April 10.
She says the humanitarian situation on the ground is "dramatically deteriorating."
Earlier Tuesday, activists said there was no sign of troop withdrawal from restive areas of Syria.
"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.
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