Moscow (AFP) - Russian President Vladimir Putin and his US counterpart Barack Obama agreed during a phone call on Monday to strengthen a Syria ceasefire brokered by their two nations, the Kremlin said.

"The leaders discussed in detail the situation in Syria, confirming in particular their intention to facilitate the strengthening of a Russian-US initiated ceasefire in this country as well as access for humanitarian aid," the Kremlin said in a statement.

Putin also stressed the need for "moderate" rebels to distance themselves from Islamic State and Al-Nusra Front jihadists and also urged the closure of the border between Syria and Turkey where "supplies of arms for extremists" are filtering across, the Kremlin said.

A landmark partial ceasefire, which was negotiated by the United States and Russia and took effect on February 27, had dramatically curtailed violence across much of Syria and raised hopes that a lasting deal could be struck in Geneva to end the bloodshed.

But fighting has surged around second city Aleppo in the last week, prompting tens of thousands of people to flee, leading the opposition to question President Bashar al-Assad's commitment to a political solution to a conflict that has displaced half of the population and killed more than 270,000 people.

White House spokesman Josh Earnest said that the phone call provided "an opportunity for the president to, once again, make the case to President Putin that he should use his influence with the Assad regime to live up to the commitments that they've made in the context of the cessation of hostilities."

"Unfortunately, we've seen that the cessation of hostilities continues to be fragile and increasingly threatened due to continued violations by the regime," he added.

Syria's opposition has postponed its "formal participation" in peace talks in protest over escalating violence, but will remain in Geneva and may continue informal discussions with mediators, the UN envoy said Monday.

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The Kremlin said both Putin and Obama had stressed the "significance" of the Geneva talks and the two also agreed that the two countries' security services and defence ministries would ramp up cooperation over Syria.

"With this end in view additional measures on how to quickly react to existing ceasefire violations will be worked out," the Kremlin added.

Obama also thanked Putin for Russia's help in securing the release of a US national, Kevin Dawes, who had been held in Syria, the Kremlin said.

The 33-year-old man -- identified by the FBI as a freelance photographer -- had been abducted in 2012 after crossing the border from Turkey.

Moscow said the Assad regime had detained him for "entering Syria illegally."