WASHINGTON (Reuters) - U.S. Secretary of State John Kerry, in a meeting last week with a small number of Syrian civilians and others, said he had lost an argument within the Obama administration to back up diplomatic efforts to end the bloodshed in Syria with the threat of using military force, the New York Times reported on Friday.

(L-R) U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov pose for a photo before a Middle East Quartet Principals Meeting during 71st Session of the United Nations General Assembly in Manhattan, New York, U.S., September 23, 2016. REUTERS/Andrew Kelly

The newspaper said it had obtained an audio recording of the 40-minute discussion that took place at the Dutch Mission to the United Nations on Sept. 22.

The approximately 20 participants included representatives of four Syrian groups that provide education, rescue and medical services in rebel-held areas and diplomats from three or four countries, the Times said.

The meeting took place days after a ceasefire Kerry had negotiated with Russia had collapsed and rebel-held areas of the Syrian city of Aleppo were coming under heavy air strikes as Moscow and Syrian President Bashar al-Assad’s government rejected a U.S. plea to halt flights.

The Times said Kerry repeatedly complained that his diplomacy had not been backed by a serious threat of military force.

“I think you’re looking at three people, four people in the administration who have all argued for use of force, and I lost the argument,” Kerry said in an audio clip posted on the Times website.

“We’re trying to pursue the diplomacy, and I understand it’s frustrating. You have nobody more frustrated than we are,” Kerry said.

The recording was made by a non-Syrian who attended the session, the newspaper reported, adding that several other participants confirmed its authenticity.Russian forces joined the Syrian war a year ago, tipping the balance of power in favor of Assad, who is also supported by Iranian ground forces and Shi’ite militia fighters from Lebanon and Iraq.

The Times said several people in the meeting pressed Kerry on what they saw as contradictions in U.S. policy.

It said one woman, Marcell Shehwaro, asked “how many Syrians” had to be killed to prompt serious action.

Kerry responded that “Assad’s indifference to anything” could push the Obama administration to consider new options, the Times said, but he also said that “any further American effort to arm rebels or join the fight could backfire.”

The Times said State Department spokesman John Kirby declined to comment on what he described as a private conversation. The State Department did not immediately respond to a Reuters request for comment.