The Syrian government has denied it carried out any chemical attack on the town. (Sign up for our free video newsletter here http://bit.ly/2n6VKPR)Secretary of State Rex Tillerson said Thursday there was "no doubt" that Syrian President Bashar Assad was responsible for a deadly poison gas attack that killed dozens of Syrian civilians, including children, and vowed a "serious response" by the U.S.&ldquo;There is no doubt in our minds that Syria and the regime under Bashar Assad were responsible&nbsp;for this attack,&rdquo; Tillerson said Thursday. &ldquo;It&rsquo;s a serious matter; it requires a serious response,&rdquo; he added when asked if military retaliation was being planned.Trump is considering military strikes against Syria in response to poison gas attack &gt;&gt;The Trump administration is weighing options, including a military attack&nbsp;that could involve limited airstrikes.Trump himself suggested to reporters aboard Air Force One on Thursday that Assad might not be able to remain in power in the wake of the strike.&nbsp;&ldquo;What Assad did is terrible. What happened in Syria is truly one of the egregious crimes and it shouldn&rsquo;t have happened. And it shouldn&rsquo;t be allowed to happen," the president said.Asked what specific steps he might take, Trump replied:&nbsp;&ldquo;I don&rsquo;t want to say what I&rsquo;m going to be doing with respect to Syria.&rdquo;Tillerson, making rare public comments after receiving Chinese President Xi Jinping at the beginning&nbsp;of&nbsp;a two-day summit in Florida, also held Russia at least partly to blame for the gas attack and, reversing a position he uttered just last week, said there was &ldquo;no role&rdquo; for Assad to continue to govern the people of Syria.&ldquo;It is important that [the Russians] consider carefully their continued support for the [Assad] regime,&rdquo; Tillerson said.Tillerson heads to Moscow next week on his first official trip there. Although the trip had already been planned, it will focus in part of Russia&rsquo;s continued support for Assad, officials said Thursday.Tillerson called his Russian counterpart, Foreign Minister Sergey Lavrov, on Wednesday to hear Russia&rsquo;s version&nbsp;of the Syria attack, a senior State Department official said. No further details were available, but Moscow has parroted Assad&rsquo;s claims that it used no chemical weapons and that the gas was probably contained in a rebel storage facility and was released in an airstrike.Last week, both Tillerson and the U.S. ambassador to the United Nations, Nikki Haley, said the U.S. had abandoned its demand that Assad be removed from power and that the only priority in the Middle East was the defeat of the Islamic State terrorist organization, which is concentrated in Syria and Iraq.Tillerson&rsquo;s comments Thursday seemed to return to the earlier, Obama-era position. He said an international political and diplomatic process to remove&nbsp;Assad was already underway.Apparently he was referring to U.N.-sponsored talks in Geneva that have made scant progress in reaching even the outlines of a political solution, much less a sustained cease-fire.