Kerry, speaking to Fox News in Doha, Qatar, became the first U.S. official to acknowledge the off-site training on the record. He would not go into detail about the training, describing it as "one part" of the U.S. effort and something that "a lot of countries" are doing.
"I think what President Obama is hoping is to build on what has already happened. The president put in place sanctions and that helps to strip some of President Assad's ability to fuel his war machine," Kerry said.
The New York Times previously reported on the effort to train Syrian opposition forces, quoting unnamed officials, but this is the first time the State Department has spoken openly about it.
"There are a lot of nations working at this," Kerry said of the effort to pressure Assad. "And so I think President Assad needs to read the tea leaves correctly."
Kerry, who is on his maiden international tour as secretary of state, cited last week's meeting in Rome where European nations joined the U.S. in offering support to the Syrian opposition. This included an additional $60 million in aid from the Obama administration and a commitment to provide non-lethal aid.
Kerry described the international sentiment as a "conviction that no nation is going to stand by while he slaughters his people with SCUD missiles, and his jets dropping bombs. And ... we've ratcheted up yet another level with the hope of convincing him and his allies that the time has come to really negotiate with the transitional government."
Kerry also discussed the ongoing investigation into the Sept. 11, 2012, terror attack on the U.S. Consulate in Benghazi. Though some in Congress are concerned the investigation has not made measurable progress over the last six months, Kerry voiced confidence in the FBI's work.
"Justice sometimes takes a while when you operate by high standards and when you need the levels of evidence that we do. But we are working at it and we will continue to work at it," he said.
Kerry revealed that he has visited with one of the survivors of the attack, whom he described as "a remarkably courageous person, who is doing very, very well." He said he also talked to the survivor's wife. Kerry could not say why the public has heard so little about the survivors of the attack.
Four Americans, including U.S. Ambassador Chris Stevens, died in the Benghazi assault.
Kerry, who in 2004 ran for president against George W. Bush largely on an anti-Iraq war message, on Tuesday praised President Obama for "getting our troops out of Iraq."
But, as the U.S. approaches 10 years since the start of the Iraq war, Kerry also said Bush "deserves credit for what the troops did when they went in."
He said the troops "did an absolutely stunning job, an extraordinary job."
Fox News' James Rosen contributed to this report.