President Obama, in his efforts to win America’s support for a punitive strike on Syria, faces the similar-yet-larger challenge of gaining international support, with Russia and China on Monday leading the international opposition.

Russia’s foreign minister, Sergey Lavrov, said the information the U.S. showed Moscow trying to prove that the Syrian regime was behind an Aug. 21 chemical-weapons attack is “absolutely unconvincing.”

Moscow is Syrian President Bashar Assad’s key ally, weapons supplier and protector at the United Nations.

The Obama administration insists Assad’s troops were behind the recent chemical attack that killed more than 1,400 people and will resume efforts Monday to win congressional support, including Obama holding a conference call with House members and meeting with Sen. John McCain, R-Ariz.

Ahead of the scheduled meeting, China said it opposes the U.S. acting alone and that any response must conform to the United Nations Charter and the basic principles underlying international relations.