US President Barack Obama said it was "time for Russia to show that it is serious" about bringing peace to Syria.

US President Barack Obama said it was "time for Russia to show that it is serious" about bringing peace to Syria.

After meeting with top aides at the Pentagon on Thursday, Obama said Russia risked casting itself as an "irresponsible actor" on the world stage.

"I'm not confident that we can trust the Russians or Vladimir Putin," Obama said. "Which is why we have to test whether or not we can get an actual cessation of hostilities."

"Russia will have shown itself very clearly to be an irresponsible actor on the world stage that is supporting a murderous regime, and will have to answer to that on the international stage."

"Russia may not be able to get there, either because they don't want to, or because they don't have sufficient influence over (Syrian President Bashar) Assad. And, that's what we're going to test," Obama said.

Meanwhile, the US President admitted that Syria had been one of the most testing issues of his presidency, which will end in January.

"I've been wrestling with this thing now for a lot of years," he said. "I am pretty confident that a big chunk of my gray hair comes out of my Syria meetings."

"There is not a meeting that I do not end by saying is there something else that we could be doing that we haven't thought of? Is there a plan 'F,' 'G,' 'H,' that we think would lead to a resolution of this issue."

After meeting with advisors at the Pentagon, Obama said the ISIL Takfiri group would "inevitably" be crushed and said the US-led coalition would continue to aggressively target the insurgents "across every front."

But "the decline of ISIL in Syria and Iraq appears to be causing it to shift to tactics that we've seen before -- an even greater emphasis on encouraging high-profile terrorist attacks, including in the United States," Obama warned.