Multiple activist groups in Syria tell CNN that dozens of people, including children, were killed in one of the deadliest suspected gas attacks in years. The activists have also blamed the attack in the rebel-held city of Khan Sheikhoun on Assad.
A US intelligence official said the attack reportedly killed 58 people, including 11 children.
"The US government is working to ascertain the truth behind this incident, but it has the fingerprints of a regime attack," the official said. "If the Assad regime was indeed responsible for perpetrating this attack, the reported casualty figures would make it the biggest incident like this since the Syrian regime sarin attack in August 2013 against the Damascus suburbs."
Two US officials separately tell CNN that it appears the regime may have used sarin, but the testing to verify that is not complete. However, this early belief is based on the high casualty count and level of injuries.
"Today's chemical attack in Syria against innocent people including women and children is reprehensible and cannot be ignored by the civilized world," Spicer said. "These heinous actions by the Bashar al-Assad regime are a consequence of the past administration's weakness and irresolution."
Spicer added: "President (Barack) Obama said in 2012 he would establish a red line against the use of chemical weapons and then did nothing. The United States stands with our allies across the globe to condemn this intolerable act."
Despite the attack, the Trump administration has said that it is up to the people of Syria to pick a leader and that their priority is not getting Assad out of office.
Secretary of State Rex Tillerson said on a trip to Turkey that the "longer-term status of President Assad will be decided by the Syrian people."
And in New York, US Ambassador to the UN Nikki Haley was even stronger about the Trump administration's decision not to push for Assad's departure.
"Our priority is no longer to sit and focus on getting Assad out," Haley told wire reporters last week, according to AFP
"Do we think he's a hindrance? Yes," she said. "Are we going to sit there and focus on getting him out? No."
McCain blames Obama -- and Trump
Republican Sen. John McCain told CNN's Alisyn Camerota there was more than enough blame to go around for Obama and Trump.
The Arizona Republican compared Trump's lack of action to that of the Obama administration, saying neither president took the situation seriously enough.
"We've seen this movie before, when Barack Obama said they would have a red line and they crossed it and he did nothing," he said on "New Day."
But McCain also had scathing criticism for Trump, saying: "Bashar Assad and his friends, the Russians, take note of what Americans say. I'm sure they took note of what our secretary of state said just the other day that the Syrian people would be determining their own future themselves -- one of the more incredible statements I've ever heard."
"I'm sure they are encouraged to know the United States is withdrawing and seeking a new arrangement with the Russians," he added. "It is another disgraceful chapter in American history and it was predictable."
McCain -- a vocal critic of Trump's foreign policy -- said the commander in chief needs to speak out against Assad and promise that his allies will pay a price for their actions against the Syrian people.
"I want to hear him say we're going to arm the free Syrian army," he said. "We're going to dedicate ourselves to the removal of Bashar al-Assad. We're going to have the Russians pay a price for their engagement. All players here are going to have to pay a penalty and the United States of America is going to be on the side of people who fight for freedom."
But McCain said he sees no clear foreign policy doctrine coming from the current administration.
"I don't see any doctrine right now. I do have great confidence in the security team around the President," he said. "I hope they will develop a strategy, stand up and give the President the advice and counsel I believe he needs and could get from that team."