Kerry and senior U.S. officials met Monday with Ahmad Jarba, president of the Western-backed Syrian National Coalition, and other opposition officials, before the coalition considers whether it will attend the Jan. 22 conference in Geneva, Switzerland, the New York Times reported Tuesday.
The coalition, based in Istanbul, Turkey, has expressed concern that its already-limited influence within Syria would fade further if it participates in a drawn-out peace conference that doesn't yield results.
The United States has tried to stress several "confidence building" measures may be instituted before the conference, such as opening humanitarian aid corridors and establishing local cease-fires.
Officials also told the Times Kerry has hinted the Obama administration could restore shipments of non-lethal military assistance to the opposition coalition that recently were cut off because of concerns that some of the aid fell into the hands of Islamist militants.
The Syrian National Coalition is to decide this week whether it will attend the Geneva peace conference aimed at establishing arrangements for a transitional administration that would govern Syria if President Bashar Assad can be persuaded to leave.
In Washington, State Department spokeswoman Jen Psaki said Kerry discussed what's at stake for the coalition, but did not say directly the United States would end aid if the opposition did not attend the conference.
"Secretary Kerry made clear privately, as he has many times publicly, that there are high stakes at play for the [opposition coalition], and that the international community strongly believes that it is in their interests and the interests of the Syrian people for them to send a representative delegation to the conference," Psaki said. "He did not indicate that the United States was planning to cut off assistance."
Kerry traveled to Rome Tuesday for meetings at the Vatican then heads to Kuwait for a meeting Wednesday of donors providing humanitarian assistance to Syria.