Munich (Germany) (AFP) - Russia must change its military targeting as it backs the Syrian regime, US Secretary of State John Kerry said Saturday, as world powers seek a cessation of hostilities in the country within a week.

"To date, the vast majority of Russia's attacks have been against legitimate opposition groups," Kerry said of Moscow's air strikes in support of President Bashar al-Assad's forces.

"To adhere to the agreement it made, we think it is critical that Russia's targeting change."

Russia has supported the Assad regime against rebel forces whom it collectively labels "terrorists". The United States has provided some support to the rebels and called for Assad to leave power.

Friday's agreement called for a "cessation of hostilities" within a week and for greatly increased humanitarian access, but it excludes the Islamic State group and al-Qaeda-linked jihadists.

"There's a lot of work to do before an effective cessation can commence," said Kerry, whose country together with Russia co-chairs a UN task force meant to find ways toward a durable cessation of violence.

"There is no way to adequately deal with the cessation of hostilities unless we do sit down and work together on every aspect of this, from the political to the humanitarian to the military also. And we are doing that now."

Kerry cautioned that "we are not approaching this with some sense of pie-in-the-sky hope".

In talks with Russia, he said, "we will work through where this targeting should take place, where it shouldn't, how we work together in order to be effective so we don't drive people away from the table.

"Because, obviously, if people who are ready to be part of the political process are being bombed, we are not going to have much of a conversation."

Kerry stressed this was the crucial moment in a five-year war that has claimed more than 250,000 lives.

"This is the moment. This is a hinge point," he said. "Decisions made in the coming days and weeks, and a few months could end the war in Syria –- or could define a very difficult set of choices for the future."

He added: "The war in Syria has now lasted for almost five years –- and shows no signs of burning itself out –- which is why we are so focused on a political track.

"If the international community and the Syrians themselves miss the opportunity now before us to achieve that political resolution to the conflict, the violence, the bloodshed, the torture, the bombing, and the anguish will continue -- so will the siren call to jihad."