Saudi Arabia on Sunday accused President Bashar al-Assad's regime and its ally Russia of numerous ceasefire violations in Syria.
"There are violations to the ceasefire from Russian and (Syrian) regime aircraft," Saudi Foreign Minister Adel Jubeir told reporters in Riyadh.
According to Syria's main opposition grouping, no fewer than 15 violations were committed by government troops and allied forces on the first day of the truce, a spokesman told reporters Sunday.
"There were 15 violations by the regime forces on day one of the ceasefire, including two attacks by (Lebanese terror group) Hezbollah in Zabadani" west of Damascus, said Salem al-Meslet, spokesman for the High Negotiations Committee.
For its part, Russia on Sunday said that opposition forces had breached the truce. It said the ceasefire had been broken nine times over the past 24 hours including by Turkey, but insisted that the deal was mostly holding.
The defense ministry said violations were committed by moderate rebels as well as "terrorist organizations."
"Over the past 24 hours, nine instances of violations of cessation of hostilities have been uncovered," the ministry said, citing its coordination center at the Hmeimim airbase in Syria.
"On the whole, the ceasefire regime in Syria is being implemented," it said of the deal brokered by Moscow and Washington which took effect from 10 p.m. GMT Friday.
"At the same time there are a number of violations by groups of 'moderate' opposition and units of international terrorist organizations." The ministry highlighted an overnight attack on the town of Tal Abyad on the border with Turkey.
A group of up to 100 fighters from the Islamic State (ISIS) stronghold of Raqa, who crossed the border from Turkey, mounted an attack on the northern part of the Tal Abyad, it said, adding they were acting in unison with other fighters.
ISIS as well as the Al Qaeda affiliated Nusra Front are not included in the ceasefire deal, which skeptics had noted would make it exceedingly difficult to implement.
"Fire from Turkish territory"
"The activities of the armed groups were supported by artillery fire from Turkish territory," the chief of the coordination center, Lieutenant General Sergei Kuralenko, said in televised remarks.
"Kurdish rebel units had pushed the fighters out of the city by morning of February 28."
Moscow said it had demanded an explanation from the United States, which leads an anti-ISIS coalition that includes Turkey. Turkey has said it is not bound by the ceasefire deal if its national security was threatened.
The Tal Abyad attack was "confirmed through several channels including representatives of the Syrian Democratic Forces," Kuralenko was quoted as saying by Russian reporters, referring to a US-backed Kurdish-Arab alliance.
According to the Syrian Observatory for Human Rights, the Kurdish forces and their Arab allies successfully pushed back a fierce ISIS offensive by Saturday night with backing from the US-led coalition.
In Latakia, Nusra Front jihadists, acting from territory controlled by moderate rebels, shelled a unit of Desert Falcons, the Russian ministry said, referring to an Iranian-backed regime force.
"As a result of the shelling there are a lot of dead and wounded among rebels and locals."
Russia also said Damascus was shelled six times Saturday, adding that the attack came from territory controlled by moderate rebels including Eastern Ghouta, east of the capital.
"At the request of the Russian center for reconciliation, Syrian government troops did not open return fire," the ministry said.
AFP contributed to this report.