Kerry says Assad won't negotiate when he can 'rub out' his opponents with chemical weapons

• NEW: Kerry rejects doubts on Syrian intelligence, asks "What are we going to do about it?"
• Russia foreign minister Sergey Lavrov says Syrian government promises to negotiate
• Lavrov blames rebels for stopping peace process
• Syrian foreign minister promises cooperation with Moscow-led effort

(CNN) -- Russia's foreign minister Monday called for international talks in Moscow to promote a peace process for Syria and avert an American military strike.

Sergey Lavrov, speaking in Moscow alongside his Syrian counterpart, blamed U.S.-backed rebels in Syria for preventing a peace conference in Geneva.

But U.S. Secretary of State John Kerry, speaking moments later in London, said it is Syrian President Bashar Al-Assad who will not negotiate as long as he is not prevented from using chemical weapons.

"If one party believes he can rub out countless numbers of his own citizens with impunity ... he will never come to a negotiating table," Kerry said.

Lavrov repeated Russian assertions that Syrian rebels may have used chemical weapons. Kerry said the rebels don't have the scientific or military capability, and repeated American claims that the rockets used in an August 21 attack near Damascus were launched from regime-controlled territory.

The Obama administration says that attack killed more than 1,400 people near Damascus on August 21.

"The evidence is powerful," Kerry said. The question now, he said, is "What are we going to do about it? Turn our backs? Have a moment of silence?"

Assad on Sunday denied again he had anything to do with the use of chemical agents, U.S. journalist Charlie Rose reported. The PBS anchor and CBS "This Morning" co-host said al-Assad told him during an interview to be broadcast Monday that Syria was prepared to retaliate if there was a military strike by the United States and its partners.

Despite American support for a military strike to punish Assad and hurt his ability to use chemical weapons, Kerry said, "The end to the conflict in Syria requires a political solution. There is no military solution."

The United Nations has said more than 100,000 people -- including many civilians -- have been killed in Syria since a popular uprising spiraled into a civil war in 2011.

Kerry said the United States still supports a future round of talks in Geneva.

Lavrov told reporters in Moscow that Syrian Foreign Minister Walid Moallem "said quite clearly Damascus is ready to participate in a positive way" in negotiations.

Lavrov said the Russian government would work with other nations to promote negotiations, "and if we can understand these contacts will help, then we can invite all those interested in the world to Moscow."

At the joint press conference, Moallem said, "For the sake of our country, we will cooperate with Russia to resolve the issue."

But Moallem, repeating the charge that Syrian rebels are dominated by terrorists, said if the United States chooses to "strike their Syrian army's abilities in favor of al Qaeda and its branches in the region ... then this is a different matter."