Clashes between rebels and Syrian government forces have been reported within two hours of a nationwide ceasefire taking effect.
The British-based Syrian Observatory for Human Rights monitoring group said rebels violated the deal backed by Russia and Turkey and had taken over a position in Hama province.
A spokesman for the Jaish al-Nasr rebel group accused government forces of violating the truce by shelling areas in two villages in Idlib province, which borders Hama.
Gunfire was earlier reported in the southern Syrian provinces of Deraa and Quneitra.
Gunfire has been reported shortly after a ceasefire deal took effect at midnight (10pm GMT), Syrian activists have said.
The London-based Syrian Observatory for Human Rights said shooting was heard in the southern Syrian provinces of Deraa and Quneitra.
The Observatory said there were no reports of casualties, adding that in other areas of Syria warring sides appeared to have ceased firing.
A new ceasefire brokered in Syria by Russia and Turkey has come into effect.
The deal, where Turkey and Russia will act as guarantors, became effective from midnight local time.
Peace talks between President Bashar al-Assad's government and rebel forces in Syria are set to take place next year in Kazakhstan.
Assad was reported by the Kremlin on Thursday to be "committed" to the truce.
Syrian President Bashar al-Assad is "committed" to observing the freshly-signed ceasefire deal in the country, according to the Kremlin.
Al-Assad signalled his intent to abide by the rules of the deal in a telephone conversation with Russian President Vladimir Putin, the Kremlin said.
The pair agreed that upcoming peace talks in Kazakhstan over the five-year conflict would be "an important step towards the ultimate resolution of the crisis".
Syria's foreign minister has said there is a "real chance" for political settlement in the war-torn country - hours after a new ceasefire deal was signed.
Walid Al-Moualem gave the clearest sign yet that the five-year conflict in Syria might have an end in sight.
But Al-Moualem, speaking after Thursday's truce was signed, warned that rebel fighters must distance themselves from terrorist networks.
He urged forces opposed to President Bashar al-Assad to put clear distance between themselves and organisations like so-called Islamic State and former Al Qaeda fighters.
"It is the duty of the factions (rebel groups) who have signed it to distance themselves from, and declare that they are not linked to, the Nusra Front or Daesh (Islamic State)," al-Moualem said on state TV.
He added that Syria would participate "with an open mind" in upcoming peace talks in Kazakhstan.
News of the latest Syria ceasefire is a "positive development," the US State Department has said.
A spokesman for the department said the US hoped the deal would be "implemented fully and respected by all parties".
"Any effort that stops the violence, saves lives, and creates the conditions for renewed and productive political negotiations would be welcome," Mark Toner said.
Turkey and Russia are set to act as guarantors to the treaty, which will come into effect at midnight on Thursday.
Turkish President Recep Erdogan has maintained that the new Syria ceasefire deal will not stop the fight against co-called Islamic State.
Erdogan welcomed the truce, signed on Thursday, where both Turkey and Russia will act as guarantors.
The 62-year-old leader said the window of opportunity created by the new ceasefire should not be wasted.
But he insisted that the struggle against terrorist groups such as so-called IS would continue.
Civilian lives might be saved and aid more easily delivered because of the new Syria ceasefire agreement, a senior envoy believes.
Staffan de Mistura, the UN's envoy to the war-torn country, said he hoped the newly-brokered deal would pave the way for productive future talks between warring parties.
Welcoming the ceasefire's announcement on Thursday, De Mistura argued that it would contribute to inclusive and productive negotiations on February 8.
A spokesperson for the Free Syrian Army Rebel Alliance has said that the ceasefire will cover the whole of the country.
They added that the opposition will abide by the ceasefire and take part in future peace talks, carried out with Russia and Turkey acting as guarantors.
The spokesperson added that future talks will be carried out in line with the Geneva process and Syrian President Bashar Al-Assad will have no say in the country's future.
However, they also added that they had not met with anyone from the Syrian regime during talks.
Syrian Kurdish groups and their allies have approved a plan for a system of federal government in northern Syria.
"The social contract draft was ratified ... and the executive committee will prepare for elections" first to regional administrations and later to a central body, an official said, without giving a date for the votes.
Opposition forces have also been given the right to name a delegation to attend peace talks in Kazakhstan which will take place within the next 30 days.