Shells have hit parts of the Syrian capital Damascus less than a day after a landmark "cessation of hostilities" came into effect.
Warring sides in the five-year conflict had accepted the United States-Russia plan for a temporary halt in fighting, which came into effect at 11am New Zealand time yesterday (midnight local time).
Under the fragile agreement, Syria's government and its enemies are expected to stop fighting so aid can reach civilians and peace talks can resume.
More than 250,000 Syrians have been killed in Syria's civil war and millions more have been forced from their homes.
Early reports said front lines were silent after the temporary ceasefire began and, apart from isolated fire, fighting appeared to have stopped in most of western Syria.
But state media reported several shells have since hit residential areas of Damascus. They did not say who had fired.
Low-level clashes have also been reported between rebel groups and government forces elsewhere.
An international taskforce chaired by the United States and Russia is to meet in Geneva to monitor violations of the landmark agreement.
The UN said it expected breaches in the temporary truce and urged restraint in curbing any new outbreaks of fighting.
"Let's pray that this works because frankly this is the best opportunity we can imagine the Syrian people has had for the last five years in order to see something better and hopefully something related to peace," said UN Syria envoy Staffan de Mistura.
The truce involves Syrian government and rebel forces, but not the so-called Islamic State group (IS) or the al Qaeda-linked Nusra Front.
Halt to air strikes
Russia halted air strikes at midnight on armed groups that had said they would abide by the temporary truce, a senior Russian defence official told reporters at a briefing in Moscow.
The UN resolution named about 30 areas in dire need of aid, including rural Aleppo and the eastern city of Deir al-Zour, which is under siege by Islamic State jihadists.
Aid has been delivered to some besieged areas of the country this year in a series of localised agreements, but the UN demands unhindered access to all Syrians in need of help.
The Syrian government and Moscow have said they will not halt combat against Islamic State and the Nusra Front.
Other rebels seen as moderates by the West say they fear this will be used to justify attacks on them.
Nusra Front, one of Syria's most powerful Islamist rebel groups, on Friday urged insurgents to intensify their attacks on President Bashar al-Assad and his allies, adding to dangers facing the agreement.
Calm greets first hours of truce
Fighting raged across much of western Syria right up until the cessation came into effect, the Britain-based Syrian Observatory for Human Rights said, but there was calm in many parts of the country shortly after midnight.
"In Damascus and its countryside ... for the first time in years, calm prevails," observatory director Rami Abdulrahman said.
"In Latakia, calm, and at the Hmeimim air base there is no plane activity," he said, referring to the Latakia base where Russia's warplanes operate.
Mr De Mistura said he intended to restart peace talks on 7 March, provided the halt in fighting largely held.
The United States said it was time for Russia to show it was serious about halting fighting by honoring a commitment not to strike Syrian groups that are part of the moderate opposition.
The cessation agreement has not been signed by the Syrian warring parties themselves and is less binding than a formal ceasefire. A halt in fighting is desperately needed to get aid to besieged areas of the country.
Some relief has got to these regions in a series of localised agreements this year, but the United Nations demands unhindered access to all Syrians in need of help.
The Red Cross called for an end to the conflict in which most regional and world powers are now involved.
"It is time for the warring parties to end this horrendous conflict and for the world powers who can influence the situation to act decisively," its President Peter Maurer said in a statement.
NZ voted on UN Security Council
New Zealand voted in favour of the UN resolution supporting an end to hostilities in Syria.
New Zealand is currently a member of the UN Security Council, which debated the Syrian situation.
New Zealand's Ambassdaor to the UN, Gerard van Bohemen, said he hoped today would mark the beginning of the end of mass violence in Syria.
But he told the meeting the process of peace negotiations - due to begin next week - must be an inclusive one.
"We think it's right that this council today has sent a message to the Syrian parties that they must participate in those negotiations on a political transition and that they must engage in those negotiations in good faith."
Previous talks in Geneva collapsed in early February after making no progress.
Mr de Mistura said he had no doubt "there will be no shortage of attempts to undermine this process".
"This will remain a complicated, painstaking process," he told the UN Security Council via videoconference from Geneva.
But he added: "Nothing is impossible, especially at this moment."
The UN Security Council also unanimously adopted a resolution drafted by the US and Russia that endorsed the truce agreement.
It urges all sides to "use their influence with the parties to the cessation of hostilities to ensure fulfilment of those commitments"
If the current agreement holds, it will be the first time a pause in Syria's five-year civil war has been negotiated by world powers.
IS and Nusra Front not part of deal
The truce involves government and rebel forces - but not IS or the al-Qaeda-linked Nusra Front. On Friday, Nusra Front urged its supporters to intensify attacks against the Syrian president, Bashar al-Assad, and his allies.
The Syrian government and Moscow have said they will not stop fighting against those groups.
Moscow's intervention in the war in September with an air campaign helped President Bashar al-Assad's forces and their allies recapture territory in Aleppo and Latakia provinces.
Rebels have advanced elsewhere including in Hama province, but fighting has largely tipped in favour of Damascus, which is also backed by Lebanese Hezbollah and Iranian fighters.
Nusra Front on Friday called for an escalation in fighting, urging insurgents to intensify their attacks, adding to the dangers facing the fragile peace agreement.
Saudi Arabia, which supports insurgent groups, has said it is willing to send its forces into Syria to fight Islamic State, and Turkey, another Assad opponent, wants ground troops deployed but has denied plans for unilateral action.
The Syrian government has said the cessation plan could fail if foreign states supply rebels with weapons or insurgents use the truce to rearm.
The main Saudi-backed opposition alliance, which has deep reservations, said it would accept it for two weeks but feared the government and its allies would use it to attack rebel factions under the pretext that they were terrorists.
The US-backed Kurdish YPG militia, which is battling Islamic State in the northeast and Turkish-backed rebel groups in the northwest, said it would abide by the plan, but reserves the right to respond if attacked.
Fighting between the YPG and Islamic State continued in Raqqa province, the Observatory said.
In the run-up to the midnight deadline, US President Barack Obama warned the Syrian government and Russia "the world will be watching".
Russian jets were reported to have intensified attacks on Syrian rebel positions on Friday.
-BBC/ Reuters/ RNZ