The attack -- which the State Department had previously warned about -- is just the latest atrocity Assad's regime is said to be responsible for in Syria's eight-and-a-half-year old war, many of which amount to "war crimes and crimes against humanity," Pompeo said Friday.
Days after the United Nations announced that Assad and the Syrian opposition agreed to finally convene a constitutional committee to begin a negotiated settlement to the war, the U.S. has been rallying Western and Middle Eastern allies and partners to hold the line of pressure against Assad.
In particular, the U.S. has tried to use funding for reconstruction, vowing to withhold any until Syria and its chief backer Russia agree to a political process. The announcement that the constitutional committee will convene in the coming weeks is a sign of that campaign's success, according to U.S. special envoy for Syria James Jeffrey, calling it a "glimmer of hope that this conflict can be ended the right way."
Pompeo provided few details about the chlorine attack, beyond that it was conducted in Latakia province and was part of Assad's larger assault on Idlib province -- the last rebel-controlled stronghold. Since April, regime forces -- boosted by Russian airstrikes -- have moved into Idlib despite a ceasefire with Turkey, which backs the rebels.
The offensive has killed over 1,000 people, according to the U.N. It has also sparked fears of a slaughter in Idlib, which houses more than 3 million civilians who've fled Assad's forces from elsewhere in Syria, and prompted concern about a new flood of refugees moving into Europe.
President Donald Trump has warned Syria and Russia of any escalation against Idlib, but has been unwilling to take actions to halt it.
In May, the State Department said it had seen "signs" of a possible chlorine gas attack, but was "still gathering information," according to spokesperson Morgan Ortagus. Months later, Pompeo declined to say how the U.S. had reached its determination.
"The United States will not allow these attacks to go unchallenged, nor will we tolerate those who choose to conceal these atrocities," Pompeo said Friday, but he later declined to preview any response.
The Trump administration has twice conducted airstrikes on the regime, in April 2017 and April 2018 after Assad reportedly used sarin gas. The Assad regime and Russia have denied Assad's forces have used chemical weapons. The U.N. chemical weapons watchdog, the Organization for the Prohibition of Chemical Weapons, or OPCW, confirmed chemical weapons were used in both attacks, but did not ascribe blame.
But Pompeo implied that military action was unlikely, given that it was chlorine and not sarin gas used. He did announce, however, that the U.S. will provide a new $4.5 million in funding to the OPCW.
"This is different in some sense in that it was chlorine, so it's a bit of a different situation," he said.
Special envoy Jeffrey later told reporters, "We're not going to tie ourselves down to any military or political, diplomatic action. ... Four people were wounded. It was four months."
Instead, U.S. officials pointed to new sanctions from the Treasury Department against one company, three of its Russian employees and five Russian-flagged ships for smuggling jet fuel into Syria for Russian forces. The fuel has "enabled" Assad's "continued bombing campaigns that destroyed numerous hospitals, schools and public spaces, resulting in civilian deaths," according to Treasury.
While Pompeo accused Assad of war crimes and crimes against humanity, the U.S. has stopped saying he "must go," instead saying that there must be a political transition in Syria and they believe he would not win free and fair elections.
Among the regime's many atrocities, Pompeo specifically called out the detention and disappearance of more than 100,000 people in Assad's prisons, including a "number of American citizens."
"We call upon the Syrian regime to release them all," he added, highlighting U.S. journalist Austin Tice, who has spent seven years in captivity.