WASHINGTON — At home and abroad, the Obama administration redoubled its campaign Sunday to build support for military action against Syria, saying it had won the backing of Saudi Arabia for a strike while still laboring to persuade a deeply reluctant Congress.

But Syria’s president, Bashar al-Assad, thrust himself into the debate as well, rejecting President Obama’s claim that his forces used chemical weapons on civilians last month. In an interview with Charlie Rose, scheduled to be broadcast on Monday, Mr. Assad warned that if Syria was attacked, it would retaliate.

With Mr. Obama scheduled to press his case on Monday in interviews with six major television networks, the prospect of a split-screen moment loomed, featuring the two main antagonists in the international debate over how to deal with Syria.

In Paris, Secretary of State John Kerry said the Saudi foreign minister, Prince Saud al-Faisal, told him that Saudi Arabia would support an American-led strike. Qatar also said it would back foreign intervention, though it did not explicitly endorse airstrikes. Mr. Kerry said he was hopeful that additional countries would indicate support for a strong response in coming days.