President Barack Obama and Russian President Vladimir Putin were at loggerheads Monday as they discussed renewed fighting in Syria, but redoubled efforts to enforce a tenuous truce.

Obama "stressed the importance of pressing the Syrian regime to halt its offensive attacks against the opposition" during a telephone call with his Russian counterpart, according to the White House.

"The two leaders committed to intensify their efforts to shore up the cessation of hostilities and affirmed the need to end attacks by all parties and ensure humanitarian access to all besieged areas," the White House said in a statement.

Spokesman Josh Earnest told reporters that the discussion was "intense".

The truce has ushered in one of the quietest eras during the five-year conflict despite occasional breaches and continued attacks from al-Qaeda's Syrian branch and Daesh -- both are excluded from the truce.

But renewed heavy fighting during the past week led to more than 35,000 fleeing camps overrun by Daesh or close to frontlines, according to Doctors Without Borders (MSF).

The medical charity said more than 100,000 people are trapped between the Turkish border and the frontline in northern Syria.

The U.S. and Russia back opposing sides in the conflict with Moscow supporting Syria's Assad regime and Washington backing the country's moderate oposition.

Talks aimed at ending the conflict broke down Monday, with opposition negotiators deferring indirect talks amid the latest violence.

More than 250,000 people have died during the Syrian conflict and millions more have been displaced.