Kerry held his first meeting with Syrian National Coalition president Ahmad Jarba at the US mission to the United Nations in New York.
The coalition is to meet UN Security Council envoys on Friday.
After the talks, Jarba said the situation is now "desperate" in Syria as he justified his call for arms. Kerry instead stressed flagging efforts to hold an international peace conference.
President Bashar al-Assad is "pursuing a military victory using indiscriminate weapons ranging from chemical weapons to cluster bombs," Jarba said in a statement on the talks.
"To deny us the right to self-defense is to risk that the regime will survive: thousands will be executed, the repression will continue without end."
The UN announced earlier Thursday that more than 100,000 people have now been killed in the conflict that started in March 2011.
Jarba, who was elected coalition president on July 6, said time is running out.
"American leadership and drive is essential to end this war and bring the democracy that the large majority of the Syrian people want," he said.
Jarba then sought to reassure the US administration by saying the coalition "is 100% committed to an inclusive democracy for all Syrians, regardless of religion or ethnicity."
The United States is currently providing humanitarian and non-lethal military aid to Syrian rebel groups but has said it will significantly expand its military assistance to vetted rebels.
Kerry said he had a "positive" meeting with Jarba but would not discuss possible US arms supplies.
"The Syrian opposition committed that they believe Geneva II is very important and they agreed to work over the course of the next couple of weeks to pinpoint the terms, the conditions, under which they think that it can work," Kerry told reporters.
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