SYRIA (CNN) -- The world turned a skeptical eye toward Syria on Thursday after a truce cast relative calm over restive cities and towns previously pounded by government forces.
"Syria is apparently experiencing a rare moment of calm on the ground," said Kofi Annan, the special envoy who brokered Syria's peace plan.
"This is bringing much-needed relief and hope to the Syrian people who have suffered so much for so long in this brutal conflict. This must now be sustained."
However, Susan Rice, the U.S. ambassador to the United Nations, said Annan told U.N. Security Council members that Syria lacks full compliance with the peace plan and that troops and heavy weapons remain in population centers despite an agreement to withdraw.
"They need to be pulled back and forces returned with their equipment to barracks," Rice said.
In Washington, U.S. Secretary of State Hillary Clinton said Syrian President Bashar al-Assad did not have the luxury to pick and choose what to implement in the peace plan.
"The Annan plan is not a menu of options," she said. "It is a a set of obligations."
White House press secretary Jay Carney called the truce tentative.
"We cannot call what's happening on the ground a full cease-fire," Carney told reporters. "A tentative cease-fire, a less-than-full cease-fire is not equal to a full implementation of the regime's obligations under the Annan plan."
Adib al Shishakly, a member of the Syrian National Council, an umbrella group of exiles, said Damascus needs to abide by all six points of Annan's peace plan.
"They diluted the whole initiative into one thing: into the cease-fire only," al Shishakly said. "What happened to the other five?"
The plan also calls for the release of detainees, allowing access into the country for humanitarian aid and international media, and respecting the rights of peaceful demonstrators. Government guns fell mostly silent, but Syrian opposition groups still reported sporadic violence, and Thursday was not a day entirely free of bloodshed. At least 21 people were killed, said the Local Coordination Committees (LCC), a network of activists across Syria.
Amid the tenuous truce, opposition groups called for peaceful demonstrations, seemingly testing whether the government would stick to its word, and the United Nations chief said the global body is working to send in international observers as soon as possible.
"Today marks a critical moment in our six-point plan for ending the violence in Syria," said U.N. Secretary-General Ban Ki-moon. "At this moment, the situation looks calmer."
Ban said he was working with the U.N. Security Council to send in international observers as promptly as possible.
"The onus is on the government of Syria to prove that their words will be matched by their deeds this time," Ban said at a news conference in Geneva. "The world is watching, however, with skeptical eyes since many promises previously made by the government of Syria have not been kept."
Anti-regime protesters came out onto the streets in several cities including Idlib, Homs, Deir Ezzor, Raqqa, Aleppo and Damascus, according to the LCC. It said troops opened fire at a checkpoint in Hama to prevent people from protesting.
Tanks were still roaming the streets, and snipers remained perched on rooftops in some cities, according to activists. CNN reports they're unable to independently verify reports of violence and deaths as the government has severely restricted access by international media.
The continued troop presence meant that al-Assad's guns could be pointed back at people within a moment's notice, and fears rose that large protests could spark violence.
Among the dead Thursday were a child and two victims of sniper bullets. The opposition group also reported resumed shelling in a neighborhood of Homs. Many Syrians and international observers questioned whether al-Assad would adhere to the cease-fire, especially after the government made it clear that it reserved the right to fire back on aggressors.
The regime has reneged on previous promises to stop violence, and al-Assad has described anti-government rebels as armed terror groups spreading mayhem in the country.
"Armed terrorist groups escalate their criminal operations in an attempt to damage Syria's stability and Annan's plan," said a headline on Syrian state-run television.
Bashar Jaafari, Syria's ambassador to the United Nations, told reporters Thursday that armed groups had eight violations of the cease-fire that resulted in several deaths Thursday. Jaafari reiterated "again and again and again" his government's commitment to a peace plan and said Syrian troops had withdrawn from cities and towns.
"The credibility of the Syrian government has been confirmed," he said, rebutting Annan's statement that Syrian troops had not withdrawn.
"He didn't say that," Jaafari told reporters.
Rice, however, told reporters she couldn't imagine how the Syrian ambassador, who wasn't in the room when Annan was speaking, could characterize accurately what the former U.N. secretary-general said.
UK Foreign Minister William Hague said Thursday's lessening of violence "is a first fragile step toward peace that needs to be strengthened and sustained."
For the most part, after months of slaughter, residents of hotspots throughout the beleaguered nation woke up to the quietest day in months.
"Interestingly, we have been only hearing the sounds of birds," said Abu Salah, a Homs resident. "However, we have seen the security forces on alert."
Annan's plan, which appeared to falter in the days ahead of its Thursday deadline, is widely seen as the last chance for a diplomatic solution to the yearlong crisis in Syria. Opposition fighters stayed in some cities to ward off attacks by government forces, activists said.
"The Free Syrian Army members remain in Homs, but there have been no clashes," activist Saif al-Arab said in reference to the army of military defectors. "They perceive their role as only to defend civilians in the face of any raids."
Just before the deadline, a tank fired mortar rounds into a residential area in Zabadani, 20 miles outside of Damascus, activists said.