Mr. Jarba called the military situation in Homs “extremely difficult,” but “not impossible.”

A leader of the Shammar tribe, which has branches in Saudi Arabia and Iraq, Mr. Jarba, 44, was trained as a lawyer and has construction, real estate and other business interests in the Persian Gulf. Twice imprisoned by Mr. Assad, he made his first visit to the United States last week to attend a closed session of the United Nations Security Council. He met on Thursday with Mr. Kerry in New York.

On July 6, Mr. Jarba, with the support of Saudi Arabia, was elected by a narrow margin as president of the National Coalition of Syrian Revolutionary and Opposition Forces, also known as the Syrian National Coalition. Saudi Arabia is one of the main backers of the resistance, which has been vying for influence with Qatar. Mr. Jarba insisted, however, that he had strong ties with ruling families throughout the Persian Gulf.

Discussing his plans to govern areas that had been wrested from Syrian government control, Mr. Jarba acknowledged that Shariah courts had been set up by Islamist rebels but said his goal was to replace them with civil courts. While saying there were no preconditions for attending the Geneva talks, Mr. Jarba said that the opposition was asking that the Assad government take "positive steps," including the release of prisoners, that the coalition could present to the Syrian public to show that attending the talks was worthwhile.

Mr. Ghalioun said the opposition had told Mr. Kerry in their meeting that such steps also needed to include an end to artillery attacks, airstrikes and missile launches by the government forces.

That, he said, prompted Mr. Kerry to ask what the resistance might do in return, an important question as the opposition coalition does not control all the rebel groups, especially extremist factions like the Al Nusra Front.

Mr. Ghalioun quipped that the opposition would renounce the use of chemical weapons, which American officials say the rebels neither possess nor can access.