President Obama is working the phones during his trip to the G-20 summit to convince skeptical lawmakers to support his call for a military strike on Syria, the White House said Thursday.

Deputy National Security Adviser Ben Rhodes told reporters in St. Petersburg, Russia, that Obama had already made five calls to a bipartisan group of senators during his stop in Sweden on Wednesday.

Obama is attending the two-day summit in Russia which began Thursday, where he is expected to press international leaders to unite behind military intervention against Syrian. But the foreign trip comes at a difficult time, with Congress engaged in a heated debate over a strike.

Obama has sought to leave his mark on the debate, telling reporters on Wednesday he was confident that Congress would back his stance to punish Syrian strongman Bashar Assad for using chemical weapons. The administration, though, risks embarrassment if lawmakers fail to fall in line behind the president.

The Senate Foreign Relations Committee on Wednesday approved a resolution authorizing U.S. military action against Syria. However, a growing number of Obama’s Democratic allies as well as Republicans are speaking out against a military strike in Syria.

House Speaker John Boehner, R-Ohio, said he backs the president’s call for Congress to authorize an attack on Syria, but many rank-and-file members of his caucus have shown little willingness to embrace a potential military conflict with little public support.

House Minority Leader Nancy Pelosi, D-Calif., has sought to rally Democratic support, but cautioned that there could be "weeks of debate" on Syria and gave no guarantee congressional Democrats would support the White House.

Ahead of a working session with the G-20 nations, Obama made his case to the international community Thursday.

"The use of chemical weapons in Syria is not only a tragedy," he said, "but also a violation of international law that must be addressed."