OXFORD, England -- U.S. Defense Secretary Ash Carter accused Russia on Wednesday of sowing seeds of global instability and questioned whether Moscow genuinely wants a viable cease-fire in Syria.

In a hard-hitting speech at Oxford University, Carter emphasized deep skepticism about Russian intentions in Syria, even as U.S. Secretary of State John Kerry considered traveling to Geneva on Wednesday for more talks with Russian Foreign Minister Sergey Lavrov. Their weekend discussions, on the sidelines of an economic summit in China, failed to produce an agreement on more cooperation in Syria.

Russia is a firm supporter of Syrian President Bashar Assad, and their joint military operation has sometimes targeted the rebels against the Islamic State of Iraq and Syria, or ISIS, backed by the Obama administration.

“Unfortunately so far, Russia, with its support for the Assad regime, has made the situation in Syria more dangerous, more prolonged and more violent. That has contributed to what President Obama this weekend called the ‘gaps of trust’ that exist between our two countries,” Carter said.

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In last weekend’s talks, top diplomats from the U.S. and Russia, as well as President Obama and Russian President Vladimir Putin, struggled to keep alive negotiations to end the bloodshed between U.S.-backed rebels and Syria’s government. Mr. Obama expressed skepticism that an unlikely alliance between rivals would yield the breakthrough needed to end the five-year-old civil war.

Carter urged the Russians to work with the U.S. toward a political transition in Syria, though he sounded less than optimistic.

“Today’s news out of Syria is not encouraging,” he said. “The choice is Russia’s to make and the consequences will be its responsibility.”

Intense fighting between Syrian government troops and insurgents in Syria’s central Hama province displaced some 100,000 people over eight days between late August and early September, the U.N.’s humanitarian agency reported Wednesday.

“Despite the progress that we made together in the aftermath of the Cold War, Russia’s actions in recent years - with its violations of Ukrainian and Georgian territorial integrity, its unprofessional behavior in the air, in space, and in cyberspace, as well as its nuclear saber rattling - all have demonstrated that Russia has clear ambition to erode the principled international order,” Carter said.

Carter accused Russia of being driven by “misguided ambition and misplaced fear.” He said Moscow understandably wants to be seen as an important world power, but is undercutting its case by undercutting the work of others.

“It lashes out, alleging that it fears for its own viability and future,” even though it should know that no country, including the U.S., is trying to constrain its potential.

He seemed to allude also to suspected Russian involvement in hacking Democratic National Committee computers in the United States and otherwise trying to influence the American presidential election.

“Let me be clear, the United States does not seek a cold, let alone a hot war with Russia. We don’t seek an enemy in Russia. But make no mistake - we will defend our allies, the principled international order, and the positive future it affords all of us. We will counter attempts to undermine our collective security. And we will not ignore attempts to interfere with our democratic processes.”

Also Wednesday, the Kremlin said the latest round of U.S. sanctions against Russia ran counter to potential cooperation on “sensitive issues” that Mr. Obama and President Vladimir Putin discussed during their meeting this week during an economic summit in China.

The Commerce Department has added 11 companies linked to the Russian arms sector to the sanctions list that the Obama administration compiled immediately after Russia’s annexation of Crimea. The move restricts the companies’ exports to the United States.