Barely two hours before the beginning of a ceasefire in Syria, the United States has warned Russia that it was time to either "put up or shut up."

US State Department deputy spokesman Mark Toner made the remarks on Friday, adding Washington had received assurances from Moscow that it would cease bombing the so-called "moderate opposition" after a truce deal reached between the two sides on Monday.

"I don't know how to put it any better than saying: 'It's put up or shut up time,'" Toner told reporters, AFP reported.

"It is time for them to show through action rather than words that they are serious about what they profess to be serious about, which is a ceasefire, a cessation of hostilities and a political process that leads to a transition," he added.

The landmark two-week ceasefire urges the “cessation of hostilities” by the warring sides of the years-long conflict.

Minutes before the midnight on Friday local time, the United Nations Security Council unanimously adopted a resolution, drafted by Russia and the United States, to endorse the truce. The agreement demanded the “cessation of hostilities” to begin at midnight Damascus time (22:00 GMT Friday).

The Syrian government has accepted the terms on the condition that military efforts against the Daesh Takfiri terrorist group and al-Qaeda-affiliated al-Nusra Front continue.

According to AFP, Russia was pounding US-backed militants in Syria in the run-up to the truce.

Syria has been gripped by foreign-backed militancy since March 2011. According to a recent report by the Syrian Center for Policy Research, the conflict has claimed the lives of over 470,000 people.