Fierce clashes erupted Tuesday as Syria's regime sent reinforcements into rebel areas and the West sought to warn Damascus of UN action unless it sticks to an agreement to pull back its armour.
Peace envoy Kofi Annan on Monday told the UN Security Council that President Bashar al-Assad had agreed to "immediately" start pulling back his forces and complete the withdrawal of troops and heavy weapons by April 10, US ambassador Susan Rice said.
The latest violence took place with Red Cross chief Jakob Kellenberger on a new mission to Syria -- his third since last year, when Damascus launched a crackdown on Arab Spring-inspired protests which the UN says has killed more than 9,000 people.
On Tuesday, monitoring groups reported that heavy fighting had engulfed opposition strongholds in the southern region of Daraa, northwestern Idlib province and areas around the capital.
Dozens of armoured personnel carriers arrived in Dael, a town in Daraa province where the uprising began in March 2011, as well as in Zabadani, a bastion of the rebellion near the border with Lebanon.
Sayyed Mahmud, an activist in Dael reached by Skype, told AFP the situation was extremely tense in the town.
"They burned down 14 houses yesterday. They are arresting people and have sent in troop reinforcements," he said.
"As part of the regime's campaign to starve the people, troops are raiding homes, destroying food stocks and equipment," he added. "For example, if they see a sewing machine, they destroy it.
"They go into bakeries and destroy the dough. There are 15-hour power cuts a day."
In Idlib, which borders Turkey, fighting was taking place on the outskirts of Taftanaz, where two civilians and one soldier were killed amid heavy machinegun fire and shelling, said the Observatory.
"Four civilians have been wounded and several homes torched," it added. "Rebels managed to disable a troop carrier and have killed or wounded a number of government troops."
In Damascus province, clashes were reported in the towns of Douma and in Zabadani, where the army was carrying out arrests and raiding homes.
The Britain-based Observatory has charged that the army is torching and looting rebel houses across the country in a deliberate campaign that could amount to crimes against humanity.
In a briefing to the 15-member Security Council on Monday, UN-Arab League envoy Annan said that "no progress" had been made on reaching a ceasefire, according to diplomats.
The former UN chief said the Security Council had to start considering the deployment of an observer mission with a broad mandate to monitor events in Syria.
Syria's UN envoy, Bashar Jaafari, confirmed the April 10 date had been agreed "by common accord" between Annan and his government.
Rice said the United States and other countries doubted Assad would commit to reining in his forces.
"Past experience would lead us to be sceptical and to worry that over the next several days, that rather than a diminution of the violence we might yet again see an escalation of the violence. We certainly hope that is not so," said Rice.
The partial implementation of Annan's six-point peace plan would include a full cessation of hostilities within 48 hours of the deadline, diplomats said.
The United States, Britain and France were working on a Security Council declaration that would put a formal stamp on the April 10 deadline.
The statement -- a draft of which will be sent to Council members on Tuesday -- would warn Assad of possible "further measures" if he reneges on the promise made to Annan, one UN diplomat told AFP.
Russia has rejected the idea of a deadline for implementation of the peace plan, with Foreign Minister Sergei Lavrov saying "ultimatums and artificial deadlines rarely help matters."
Moscow, a Soviet-era ally of the Assad regime, said only the Security Council, where it wields veto power, could put any time restrictions on Syria's compliance with the peace plan.
Kellenberger, the president of the International Committee of the Red Cross, said he would meet officials including Syrian Foreign Minister Walid Muallem and examine measures for a daily two-hour humanitarian ceasefire -- a condition set out in the Annan plan.
"I am determined to see the ICRC and the Syrian Arab Red Crescent expand their presence, range and scope of activities to address the needs of vulnerable people," he said in a statement ahead of his two-day trip to Syria.
Besides the humanitarian ceasefire, Annan's plan also calls for an inclusive Syrian-led political process, the right to demonstrate, and the release of people detained arbitrarily.