Secretary of State Rex Tillerson said Thursday that steps are "underway" to organize an "international community effort" to oust Syrian President Bashar Assad, a major policy shift triggered by the latest chemical weapons attack in the country's ongoing civil war.

"With the acts that he has taken, it would seem that there would be no rule for him to govern the Syrian people," Tillerson told reporters. "The process by which Assad would leave is something that I think requires and international community effort, both to first defeat ISIS within Syria, to stabilize the Syrian country to avoid further civil war, and then to work collectively with our partners around the world through a political process that would lead to Assad leaving."

Tillerson added that "those steps are underway" already.

It's a marked shift from last week, when Tillerson told reporters in Turkey that Assad's future would be up to the Syrian people — an apparent departure from former President Barack Obama's stated position that he had to leave power. And it adds a new layer of complexity to Tillerson's upcoming trip to Moscow, as Russia has provided Assad with major military and diplomatic support.

"It is very important that the Russian government consider carefully their continued support for the Assad regime," Tillerson said.

Russian President Vladimir Putin's team signaled that Assad might not enjoy "unconditional support" from them, as they disclaimed responsibility for the Syrian gas attack. "It is not correct to say that Moscow can convince Mr. Assad to do whatever is wanted in Moscow," Putin spokesman Dmitry Peskov said. "This is totally wrong."

One critical test is whether the Russians will concur with the U.S. assessment that Assad's regime carried out the attack. Russian Foreign Minister Sergey Lavrov agreed in 2013 that the Russians would support retaliation against the Assad regime in the event that his regime was proven to use chemical weapons. But the Russians have denied that he was responsible for every ensuing use of gas in the Syrian civil war.

"There is no doubt in our minds and the information we have supports that the Syria, the Syrian regime under the leadership of President Bashar al-Assad, are responsible for this attack," Tillerson said.