Syrian troops continue to pound rebel strongholds with deadly fire on Wednesday while the regime sought to reassure an increasingly sceptical world that it is committed to a week-old ceasefire.
As UN observers were greeted by hundreds of anti-regime protesters on the streets of Damascus, Washington said their mission was not being given the necessary freedom to properly monitor a halt to hostilities.
"We have a very small number of observers now on the ground and it seems that small number is having difficulty with the freedom that we all expected and that is required," said Susan Rice, US ambassador to the United Nations.
White House spokesman Jay Carney said the latest violence shows the "insincerity" of the regime of President Bashar al-Assad.
"It's another indication of the apparent but not entirely unexpected insincerity of the Assad regime when it promises to abide by the elements of the (special envoy) Kofi Annan plan, ceasefire and withdrawal. We will work with our allies and partners on next steps," he said.
US Secretary of State Hillary Clinton warned Assad of tougher measures if he squanders his "last chance" by failing to implement envoy Kofi Annan's peace plan six days after it came into force.
"It is obviously quite concerning" that, while UN observers are starting to deploy in Syria, the "guns of the Assad regime are once again firing in Homs, Idlib and elsewhere," Clinton said in Brussels.
Videos posted online showed a crowd surrounding two UN vehicles in the suburb of Irbin, waving revolutionary flags and carrying signs that read: "Closely observe how Bashar is exterminating his people."
UN chief Ban Ki-moon was due to report to the Security Council later on Wednesday on progress made by the advance team of seven observers.
He has made it clear the UN mission could not go forward if Damascus does not cooperate and guarantee the observers safe access across the country.
The team arrived in Damascus on Sunday and is to be expanded to 30 in the coming days.
Approved under a UN Security Council resolution, it is to be reinforced in the longer term with up to 250 international monitors, but this will require a new resolution.
Damascus said it was close to agreeing a protocol paving the way for the observers to fan out across the country to monitor the truce aimed at halting the violence that monitors says has killed more than 11,000 people.
"We are about to finalise it," foreign ministry spokesman Jihad Makdisi told AFP. "Discussions with UN observers have been constructive and both parties agree on 90 percent of the points."
Washington earlier warned that hopes of a larger mission in Syria were being jeopardised by the persistent violence, which saw at least 14 people killed on Wednesday, seven of them civilians, according to monitors.
Three civilians were killed as regime forces launched a fresh bombardment of a rebel neighbourhood of the flashpoint central city of Homs, said the Syrian Observatory for Human Rights.
Four others, including a nine-year-old, were killed elsewhere, the Britain-based watchdog added.
Seven government troops were also killed in a roadside bomb on the outskirts of the northwestern town of Idlib, where fighters of the rebel Free Syrian Army have been active.
France said 14 foreign ministers would attend a meeting on Syria in Paris on Thursday to send a strong message to the Assad regime to implement the peace plan.
"The obstacles to the UN observers' mission that Damascus is putting in place and the Syrian regime's continued repression, contrary to its commitments, call for a strong reaction from the international community," French Foreign Minister Alain Juppe said in a statement.
Juppe said the foreign ministers of Germany, Jordan, Morocco, Qatar, Saudi Arabia and the United States would be among those taking part.
Another senior UN diplomat said that if Damascus did not complete the protocol by the end of the week then the Security Council could not allow the full mission of 200-250 observers envisaged by UN-Arab League envoy Kofi Annan.
He said there was a risk the Syrians would not agree to give the monitors "full, unimpeded and immediate freedom of movement and access".
Damascus ally Moscow has pointed the finger at the rebels, accusing them of trying to provoke violence in a bid to torpedo Annan's plan and pave the way for foreign military intervention.
"There are plenty of those who would like to see Annan's plan fail in hopes of then demanding other options -- primarily meaning the use of (outside) force," Foreign Minister Sergei Lavrov told reporters.
Russia and China courted Western and Arab anger by vetoing two Security Council resolutions that would have blamed Assad's regime for the violence.
But they have backed Annan's peace efforts and voted in favour of Saturday's text approving the observer mission.
Syrian Foreign Minister Walid Muallem was in Beijing on Wednesday for talks with his counterpart Yang Jiechi.
"Muallem ... said Syria would continue to ... respect and implement Annan's six-point proposal," the Chinese foreign ministry said.