LONDON — On the eve of a meeting with the Syrian opposition, Secretary of State John Kerry said Tuesday that the Obama administration was weighing “stepped-up” efforts to support the rebel fighters, and that such proposals had been “front and center” in administration discussions over the past week.

Mr. Kerry said that the Obama administration still favored a diplomatic solution in which President Bashar al-Assad would hand over power to a transitional government, but that additional pressure on the Syrian leader was needed.

“The problem is you can’t get there if President Assad is unwilling to decide that he should transfer that authority, and that’s the current situation,” Mr. Kerry said. “So we are left with no choice but to try to find ways to get him to think differently about what lies in the future.”

Mr. Kerry made the remarks at a news conference at Ben-Gurion International Airport in Israel as he completed three days of meetings with Israeli and Palestinian officials.