"It has been Russian support for the Assad regime over the past months, and most recently in the siege on Aleppo, that has exacerbated, intensified the conflict," State Department deputy spokesman Mark Toner told reporters yesterday.

Toner's comments came after Russian Prime Minister Dmitry Medvedev warned that any move by Gulf nations to send troops to support the rebels in Syria would risk the outbreak of a "new world war."

The State Department spokesman said that Moscow, with its air strikes begun on February 1 over northern Syria, had "put the political process in jeopardy" as world powers try to put a stop to a ferocious civil war that has dragged on for nearly five years.

Foreign ministers gathered in Munich struggled to make any headway today on efforts to obtain a ceasefire. US Secretary of State John Kerry was among those present, along with his Russian counterpart Sergei Lavrov.

Kerry has "spoken to the fact that given the disparate groups on the ground in Syria, the different factions and the different elements on the ground fighting each other, that this could worsen and could become a broader conflict," he added.

In response to a question about Medvedev's warning, Toner said: "If that is Russia's concern, then they should look at what they're doing to support the Assad regime."

Read more: World powers agree to implement 'cessation of hostilities' in Syria

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