Osama Abu Zaid, a spokesman for the Free Syrian Army rebel alliance, shows the text of the agreement about a ceasefire between Syrian opposition groups and the Syrian government during a news conference in Ankara, Turkey, December 29, 2016. [Photo/Agencies]

Russian President Vladimir Putin, a key ally of Syrian President Bashar al-Assad, announced the ceasefire on Thursday after forging the agreement with Turkey, a longtime backer of the opposition.

Monitors and a rebel official reported clashes between insurgents and government forces along the provincial boundary between Idlib and Hama, and isolated incidents of gunfire further south less than two hours after the truce began. Warring sides appeared to have stopped firing in many other areas, however.

Russian Foreign Minister Sergei Lavrov said the United States could join the peace process once President-elect Donald Trump takes office on Jan. 20. He also wanted Egypt to join, together with Saudi Arabia, Qatar, Iraq, Jordan and the United Nations.

A number of rebel groups have signed the agreement, Russia's Defence Ministry said. Several rebel officials acknowledged the deal, and a spokesman for the Free Syrian Army (FSA), a loose alliance of insurgent groups, said it would abide by the truce.

One FSA commander was optimistic about the truce deal, the third serious attempt this year at a nationwide ceasefire.

"This time I have confidence in its seriousness. There is new international input," Colonel Fares al-Bayoush said without elaborating.

Syria's civil war, which began when a peaceful uprising descended into violence in 2011, has resulted in more than 300,000 deaths and displaced over 11 million people, half its pre-war population.

The ceasefire, in the waning days of President Barack Obama's administration, was the first major international diplomatic initiative in the Middle East in decades not to involve the United States.