Syrian forces on Wednesday attacked rebel bastions as United Nations chief Ban Ki-moon urged President Bashar al-Assad to "immediately" implement a UN-Arab League peace plan he reportedly accepted.
The United States accused the Syrian leader of failing to fulfill a pledge to respect the plan.
"Assad has not taken the necessary steps to implement" the peace plan crafted by former UN secretary general Kofi Annan, State Department spokeswoman Victoria Nuland told reporters in Washington.
China and Russia also upped the pressure, urging both their ally Syria and the opposition to honour commitments to halt armed conflict, which has claimed thousands of lives since it first erupted in March 2011.
Moscow strongly urged the Syrian opposition to "follow the example" of the Damascus regime in supporting Annan's mediation efforts to stop the bloodshed.
And in Baghdad, Arab foreign ministers thrashed out a resolution on Syria to be debated at a landmark Arab League summit on Thursday, even as Damascus warned it would not abide by any of its initiatives.
On the ground, Syrian forces backed by tanks attacked the central town of Qalaat al-Madiq and other areas on Wednesday, sparking clashes which cost at least 21 lives, the Syrian Observatory for Human Rights said.
The monitoring group said four civilians were killed in shelling, while five rebel fighters and four soldiers died in fierce clashes in Qalaat al-Madiq and surrounding villages.
The army also tried to storm other rebel positions across the country, including in northwest Idlib, in central Homs and in the southern province of Daraa, the Britain-based group said.
It said troops entered the town of Qalaat al-Madiq, in Hama province, just after dawn following a 17-day barrage of shelling and heavy gunfire to root out rebels. The army, however, was not in full control of the town.
The army's offensive is part of a drive by the regime to claim control of key rebel strongholds as it tries to crush an unprecedented revolt.
Fierce clashes were also reported in the province of Daraa, cradle of the year-long revolt that has left more than 9,000 people dead according to the United Nations.
The Arab summit in Baghdad will stop short of calling for Assad to quit or discuss arming his foes, both sharply divisive issues, Iraq's Foreign Minister Hoshyar Zebari said on the eve of the meeting of the region's leaders.
Qatar, which wanted a hard line to be adopted against Assad, is to send a low-level representative to the summit, in a show of displeasure with the attitude of its Iraqi hosts.
Signup to our newsletter and follow us on Facebook and Twitter!
The UN chief, who was to attend the summit, urged the Syrian president "to put commitments into immediate effect. There is no time to waste."
He expressed concern at the continued bloodshed but welcomed Syria's acceptance of the six-point plan put forward by Annan as an "important initial step" towards ending the bloodshed.
Ban said he would "meet key (Arab) leaders in Baghdad to discuss how the UN and Arab League states can work together in helping the joint special envoy's diplomatic efforts to get the six-point proposal implemented."
In Washington, Nuland said that the United States was concerned over "arrests and violence continuing in Syria today," vowing to "keep the pressure" on the Syrian leader.
"We will judge him on his actions, not his promises," the State Department spokeswoman said.
Annan said on Tuesday that Assad's government had accepted his plan, a move cautiously welcomed by Western nations.
The plan calls for a commitment to stop all armed violence, a daily two-hour humanitarian ceasefire and media access to all areas affected by the fighting in Syria.
The plan also calls for an inclusive Syrian-led political process, a right to demonstrate, and the release of people detained arbitrarily.
A copy of the draft resolution being debated by Arab foreign ministers in Baghdad urges the Syrian regime to "immediately stop all actions of violence and killing, protect Syrian civilians and guarantee the freedom of peaceful demonstrations for achieving demands of the Syrian people."
The text, obtained by AFP, also calls on the Syrian government and all opposition factions "to deal positively with the envoy (Annan) by starting serious national dialogue."
But Syrian foreign ministry spokesman Jihad Makdissi said Damascus will reject any initiative stemming from the Arab summit. "Syria will not cooperate with any Arab League initiative at any level," he said.
"Since its suspension from the Arab League, Syria has been dealing with member states on a bilateral level," he said in a statement.
The 22-member pan-Arab body in November voted at an extraordinary meeting to suspend Syria until Assad implements an Arab deal to end the crackdown on dissent.
Western powers cautiously welcomed the statement by Annan's camp that Assad had accepted his peace plan.
"Given Assad's history of over-promising and under-delivering, that commitment (to Annan) must now be matched by immediate actions," US Secretary of State Hillary Clinton said.