White House announces presidential phone call ahead of ceasefire 'moment of opportunity,' admits it will be 'difficult to implement.'

US President Barack Obama called Russia's Vladimir Putin on Monday to discuss the terms of a ceasefire to take effect in Syria on February 27, the White House said, cautioning that the road ahead would not be easy.

"This is a moment of opportunity and we are hopeful that all the parties will capitalize on it," White House spokesman Josh Earnest told reporters as he revealed the call between the two leaders.

Earnest, however, warned: "This is going to be difficult to implement."

In a statement about the call, the White House said Obama "emphasized that the priority now was to ensure positive responses by the Syrian regime and armed opposition as well as faithful implementation by all parties in order to alleviate the suffering of the Syrian people, galvanize the UN-led political process, and focus on defeating ISIL."

ISIL is another acronym for the Islamic State (ISIS). Both ISIS and Nusra Front, a Syrian affiliate of Al Qaeda, are not included in the ceasefire, complicating further the implementation.

Washington and Moscow on Monday announced plans for a cessation of hostilities in war-torn Syria that would go into effect on Saturday, as fierce clashes raged in the country's north.

AFP contributed to this report.