Since becoming secretary of state, Mr. Kerry has been perhaps the most aggressive official inside the Obama administration calling for military action in Syria and the arming of Mr. Assad’s opponents, say Arab diplomats and Syrian opposition leaders.

His predecessor, Hillary Clinton, also advised Mr. Obama to intervene militarily, according to current and former U.S. officials. But Mr. Kerry seems to have more leeway to pursue foreign-policy initiatives than Mrs. Clinton, possibly because the White House doesn’t view him as a political rival.

Najib Ghadbian, a leader of the Syrian Opposition Coalition, the rebels’ main political body, held meetings with Mr. Kerry earlier this year and says he was impressed by the diplomat’s aggressive stance against Mr. Assad. Mr. Ghadbian says the Syrian rebels have been disappointed by the failure of the U.S. so far to deliver arms to the rebels based along Syria’s Turkish and Jordanian borders, but he doesn’t fault Mr. Kerry.

“Clinton’s head was in the right place, but Kerry seems to be much more engaged,” said Mr. Ghadbian. “Maybe she didn’t want to take chances for political reasons, but Kerry doesn’t seem to have those political calculations.”