ISIS can be defeated within "months" of a ceasefire between Syria's government and moderate rebels, U.S. Secretary of State John Kerry said Thursday, urging the world to come together behind a peace strategy recently hashed out by the United States, Russia and other countries.

Kerry said air power alone won't be enough. The campaign will require ground forces, too, he said at a European security conference. He later specified those would be local Syrian and Arab forces, not Western troops.

Kerry has been spearheading international efforts to broker an end to fighting between Syrian President Bashar Assad's government and non-terrorist opposition forces, and a political transition process involving elements of each side.

Assad's biggest backer

The thinking is that peace between the government and moderates would allow the international community to focus military efforts exclusively on defeating Islamic State in Iraq and Syria (ISIS), al-Qaeda's Syria affiliate and other extremist groups.

Iran and Saudi Arabia have endorsed the process. Iran, along with Russia, is Assad's biggest backer. Saudi Arabia and other Sunni monarchies in the Persian Gulf have provided much of the assistance to the rebels.

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Children play in Douma, a rebel-held neighbourhood in Damascus. Kerry said Thursday a ceasefire in Syria can defeat ISIS within months. (Bassam Khabieh/Reuters)

Diplomats hope to start direct discussions between Syria's government and the opposition in the next few weeks. December talks in New York have been mooted.

Kerry said a political transition would be a boon for everyone, allowing ISIS to be "eliminated within a matter of months."

The U.S. has spoken generally of degrading and eventually destroying the group. It has tried to avoid timelines.

Lavrov to meet Turkish minister

On the sidelines of the Organization for Security and Co-operation event in Belgrade, Serbia, Kerry met Russian Foreign Minister Sergey Lavrov in a bid to smooth out tensions that have simmered since Turkey downed a Russian warplane last week. Turkey and the U.S. say the plane crossed into Turkish territory from Syria.

Lavrov was to meet later Thursday in Belgrade with Turkish Foreign Minister Mevlut Cavusoglu. It will be the first such face-to-face conversation between high-ranking Russian and Turkish officials since the Nov. 24 incident.

Co-operation between Turkey and Russia is seen as critical for any international effort in Syria.

Underscoring the difficulty of restoring previously warm ties, Russian President Vladimir Putin accused Turkey of buying oil from the Islamic State group on Thursday. He said Turkey's downing of the jet was a "treacherous war crime."