BEIRUT — The main Syrian opposition group said Saturday it is ready “in principle” to implement a provisional truce, slamming Russia and the Syrian government after a deadline set for a temporary cessation of hostilities passed.

The Saudi-backed group, known as the High Negotiations Committee, said any potential truce would require the Syrian government to first lift blockades from rebel-held communities and release thousands of detainees.

The statement followed a meeting among opposition groups held in the Saudi capital, Riyadh, on Saturday to discuss the situation after the passing of a deadline set by world powers, including Russia and the U.S.

A Feb. 12 meeting in Munich of 18 nations supporting opposing sides in Syria’s five-year civil war agreed to bring about a cessation of hostilities within a week to allow for peace talks to resume in Geneva. But the truce never took hold amid intense fighting, including a massive Russian-backed government offensive near the Turkish border.

U.N. envoy Staffan de Mistura told the Swedish newspaper Svenska Dagbladet on Friday that he cannot “realistically” get the parties in the Syrian conflict back to the table by Feb. 25 as he had hoped.

U.S. Secretary of State John Kerry spoke with Russian Foreign Minister Sergey Lavrov on Saturday to discuss the progress made by two U.N. task forces meeting in Geneva this week to discuss humanitarian assistance and a cessation of hostilities, according to State Department spokesman John Kirby.

Although the details “are not yet fully agreed upon,” Kerry expressed his hope that a full cessation of hostilities could be achieved in the shortest timeframe possible, Kirby said.

The HNC said any truce must include all parties to the conflict, notably Russia and Iran, key supporters of President Bashar Assad’s government. Russia has said that it would continue to strike at “terrorists” in Syria even during a cease-fire.

“The deadline set in Munich for a cessation of hostilities has passed without response from Russia or the regime, who show disdain for the international community and disregard for the lives of Syrians,” said HNC spokesman Salem Al Meslet in a statement.