“I don’t rule out anything right now,” Mattis responded when asked by reporters Monday about the possibility of the U.S. launching airstrikes against the Assad regime in the wake of this weekend's attack in Douma.
Mattis made his comments at the top of meeting at the Pentagon with Sheikh Tamim Bin Hamad Al-Thani, the Emir of Qatar.
“The first thing we have to look at is why chemical weapons are still being used at all," Mattis added.
"Russia was the framework guarantor of removing all the chemical weapons and so working with our allies and partners, from NATO to Qatar and elsewhere we are going to address this issue," he said.
How the administration will react to the apparent chemical attack in Douma will be a topic of discussion at a National Security Council meeting to be held Monday at the White House, a U.S. official told ABC News.
Another official said it was possible that President Donald Trump could be presented with U.S. military options at the meeting.
Speaking to reporters before a morning Cabinet meeting, Trump condemned the suspected chemical attack in Douma in no uncertain terms, calling it a "heinous attack on innocent Syrians with banned chemical weapons."
"We'll be making some major decisions over the next 24 to 48 hours," he said. "We are very concerned. When a thing like that can happen, this is humanity, we're talking about humanity. It can't be allowed to happen."
Last April, the Trump administration launched 59 Tomahawk missiles against a Syrian airbase, following a chemical weapons attack in Khan Sheikhoun. Those missiles were launched from U.S. Navy destroyers in the Mediterranean Sea.
If the administration decides to launch Tomahawk missiles again, there's only one Navy ship in the Mediterranean region that could do it: the USS Donald Cook.
On Monday, the Navy announced the ship was leaving Larnaca, Cyprus after a port call. A U.S. defense official told ABC News that the Cook's departure on Monday was previously scheduled and not related to possible military action in Syria.
While U.S. intelligence is still assessing the circumstances of the attack in Douma, a State Department official said the victims' symptoms, "reported by credible medical professionals and visible in social media photos and video, are consistent with an asphyxiation agent and of a nerve agent of some type."
On Sunday night, the Pentagon issued a statement denying reports from Syrian state media claiming that the U.S. had attacked the T-4 airbase in western Syria.
"At this time, the Department of Defense is not conducting air strikes in Syria," said the statement. "However, we continue to closely watch the situation and support the ongoing diplomatic efforts to hold those who use chemical weapons, in Syria and otherwise, accountable."
A third U.S. official confirmed to ABC News that Israeli aircraft conducted that strike, and that the Israelis notified the U.S. in advance. The Israeli military has declined to comment.
ABC News' Elizabeth McLaughlin and Conor Finnegan contributed to this report.