BERLIN — Secretary of State John Kerry said on Monday that the Obama administration has been considering new steps to increase support for the Syrian opposition and hasten the departure of President Bashar al-Assad and that some of them would be decided at an international conference in Rome this week.

“We are determined that the Syrian opposition is not going to be dangling in the wind wondering where the support is or if it’s coming,” Mr. Kerry said at a news conference in London. “And we are determined to change the calculation on the ground for President Assad.”

Mr. Kerry’s comments came amid diplomatic maneuvering and an unusual White House intervention over the Rome meeting, scheduled for Thursday.

After the Syrian opposition signaled that it would boycott the Rome conference to protest what it sees as negligible help from Western nations, Vice President Joseph R. Biden Jr. and Mr. Kerry called Moaz al-Khatib, the leader of the Syrian opposition coalition, and persuaded him to attend.