But Kerry, who joined British Foreign Secretary William Hague at a news conference in London, said he didn't expect Assad to do that.
In a strong rebuttal to remarks by Assad, Kerry said there is very compelling evidence that the Syrian regime used chemical weapons against his own people.
In an interview by Charlie Rose, slated to air Monday on CBS "This Morning," Assad argued that the evidence Kerry has disclosed about chemical weapons use amounts to a "big lie" that resembles the case for war in Iraq that then-Secretary of State Colin Powell made to the United Nations over a decade ago.
Asked to respond, Kerry told reporters in London that he would be confident to take evidence that the U.S. intelligence committee has gathered into any courtroom.
"What does he offer?" Kerry asked rhetorically of Assad. "Words that are contradicted by fact."
Kerry arrived in London Sunday afternoon from Paris where he held talks with the foreign ministers of Egypt and Saudi Arabia and members of the Arab League with respect to the new set of Israeli-Palestinian peace talks. He was returning to Washington later Monday to continue to lobby Congress to authorize a military strike against Assad's regime.
Kerry also said that the British Parliament's decision against endorsing a military strike in Syria would not damage U.S.-British relations. "Our bond is bigger than one vote — bigger than one moment in history," Kerry said.
Kerry, at another point, referenced the roughly 50 protesters outside the British foreign ministry who were chanting "One, two, three four, we don't want another war!" But he said he thinks it's important for nations to "stand up for humanity."
"This is a humanitarian catastrophe of global proportions," he said.
Kerry said the U.S. realizes that there is no military solution to conflict, but that a political resolution will not be possible if the Assad regime is allowed to continue killing the Syrian people without consequences.
"The evidence is powerful and the question for all of us is what are we going to do? Turn our backs? Have a moment of silence?" Kerry asked.
He said Assad's credibility is in question.
"He sends SCUD missiles into schools," Kerry said.
Hague agreed, saying: "Let's not fall into the trap of believing every word that comes out of this man's mouth."
"We know that his regime gave orders to prepare for a chemical attack. We know they deployed forces," he said. Kerry said Washington also "knows where the rockets came from and where they landed ... and it was no accident that they all came from regime -controlled territory and all landed" in opposition-held territory.
Meanwhile, Russian and Syrian foreign ministers said Monday they planned to push for the return of United Nations inspectors to Syria to continue their probe into the use of chemical weapons.
Lavrov said after Monday's talks with his Syrian counterpart Walid al-Moallem that Moscow will continue to promote a peaceful settlement and may try to convene a gathering of all Syrian opposition figures who are interested in peaceful settlement. He said a U.S. attack on Syria would deal a fatal blow to peace efforts.
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