Syrian opposition leaders on Thursday pressed US Secretary of State John Kerry to arm rebel groups and said there could be no peace talks while government forces kill civilians.
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.
"Until the regime has been forced to accept a political solution, we must have the means to protect ourselves and defend civilians.
"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.
He added that the coalition "fully understands American concerns about extremism and the possible diversion of military assistance."
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.
The United States and Russia agreed in May to press for a follow-up to a meeting held in Geneva in June last year which set out a plan for a transition process in Syria.
Divisions among Syrian opposition groups and hurdles thrown up by Assad's government have blocked efforts to call a new meeting, according to diplomats.
Kerry said however he felt "very positive" about the talks with the opposition. "There is a strong feeling that Geneva is important and we will work it out, " he added.
Burhan Ghalioun, a senior coalition member who was in the delegation, told AFP there could be no talks with the government while Assad forces are killing civilians.
"In principle nobody is against Geneva II, but we cannot enter into talks while the regime continues to kill hundreds every day and use arms of mass destruction," said Ghalioun.
The Syrian coalition leaders, who held talks with French President Francois Hollande in Paris on Wednesday, later met with UN ambassadors from Europe and other regional groups.
They will meet ambassadors to the 15-member UN Security Council on Friday. The envoys will include representatives of Russia, Assad's main international ally.