The United States and Russia are pushing for face-to-face talks between representatives of the Assad government and its opponents to establish a transitional authority for Syria.

Russian Deputy Foreign Minister Sergei Ryabkov said Monday that the United States is not putting enough pressure on the Syrian opposition to participate in the international talks and to drop its demand for President Assad's exit.

The Syrian government has said it is willing to attend such a conference in principle. But the main opposition coalition has rejected the idea, saying talks are meaningless while Syrian, Hezbollah, and Iranian forces commit alleged atrocities against the Syrian people.

Kerry says Russia's planned sale of advanced missile technology to Assad forces could put prospects for a peace conference at risk.

He says U.S. Undersecretary of State for Political Affairs Wendy Sherman will travel to Geneva Tuesday to meet with Russian counterparts "to continue to lay the groundwork for the possibility of a negotiated resolution."