Story highlights US and Russia to cooperate on fight against al Nusra front

Proposal had been met with skepticism in the Pentagon

Washington (CNN) Secretary of State John Kerry said Friday that the U.S. and Russia had agreed to cooperate in Syria against the al Nusra Front, al Qaeda's Syrian branch, in an effort to "restore the cessation of hostilities, significantly reduce the violence and help create the space for a genuine and credible political transition" in Syria.

But Kerry, appearing alongside Russian Foreign Minister Sergey Lavrov in Moscow, declined to provide details of the cooperation, saying "the concrete steps that we have agreed on are not going to be laid out in public in some long list because we want them to work."

Proposals to deepen military cooperation with Russia in Syria have sparked a rift at the highest levels of the Obama administration, with the Pentagon openly challenging the idea that Russia could be trusted to uphold its end of the bargain.

Kerry seemed to address this skepticism in his Friday remarks.

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