An internationally brokered ceasefire in Syria took effect at sundown Monday (September 12th), but there was deep skepticism over whether it would take hold with opposition forces yet to formally sign on, AFP reported.

The ceasefire deal, announced Friday after talks between Russia and the US, aims to bring a temporary end to the fighting and provide crucial aid to hundreds of thousands of civilians.

Under the deal, an initial 48-hour ceasefire began at 7 p.m. local time, halting fighting in areas not held by extremist groups such as the "Islamic State of Iraq and the Levant" (ISIL).

Aid deliveries to the country's many besieged and "hard-to-reach" areas were set to simultaneously begin, with government and opposition forces ensuring unimpeded humanitarian access.

The ceasefire will be renewed every 48 hours and, if it holds for a week, Russia and the US will begin jointly targeting extremist groups including ISIL and Fateh al-Sham Front.

But Syria's main opposition coalition on Monday demanded guarantees before endorsing the deal, saying it was unclear how the deal defined the "terrorist" groups that will be targeted, and what the response would be to truce violations.

In a further sign of the deal's fragility, Syria's President Bashar al-Assad on Monday vowed to retake the whole country from "terrorists".