According to de Mistura, initial reports indicated that the situation calmed down around the Syrian capital in the early hours of Saturday, although there was unconfirmed information of potential breaches.
"On cases of breaches, a military response should be, according to the [International Syria Support Group] task force, the last resort…and should be proportionate," de Mistura stressed on Friday.
Permanent Representative of Syria to the United Nations Bashar Jaafari said at the UN Security Council on Friday that the Syrian government forces reserve the right to respond to any violations of the ceasefire.
According to the UN envoy, 97 armed groups in Syria, plus the government and all the major regional and international stakeholders have expressed willingness to accept the framework of the cessation of hostilities. However, violations of the ceasefire are likely, as in any other conflict, de Mistura said.
"The UN is not part of addressing the incidents [of ceasefire breaches]," de Mistura emphasized, explaining that the task force of the International Syria Support Group (ISSG), as well as its co-chairs (Russia and the United States) are responsible for "addressing the infringements that are likely to take place in order to ensure that they are contained."
The UN Security Council unanimously adopted a resolution endorsing the Russia-US agreement on the cessation of hostilities in Syria on Friday, demanding that all parties strictly comply with the agreement.
Syria has been mired in civil war since 2011, with government forces loyal to Syrian President Bashar Assad fighting against numerous opposition factions and extremist groups, such as ISIL (also known as Daesh), which is banned in a range of countries, including Russia and the United States.
Russia has been launching airstrikes against IS targets in Syria at the request of Assad since late September 2015. A US-led international coalition has been conducting airstrikes against ISIL in Syria since September 2014, without the approval of Damascus or the UN Security Council.