Mattis told reporters that Washington's military strategy in Syria had not changed even after its retaliatory missile strikes on a Syrian air base, noting "our priority remains the defeat" of the Islamic State group.
"There is no doubt the Syrian regime is responsible for the decision to attack and for the attack itself," Mattis said.
"In response to the attack," US national security officials came up with "diplomatic and military options" presented to President Donald Trump, said Mattis, who added that he also spoke with Washington's allies.
In his first press conference since becoming defense secretary, Mattis reiterated warnings that further use of chemical weapons will be met with additional US action.
"If they use chemical weapons, they are going to pay a very, very stiff price," Mattis said.
An important question remains whether the United States would take further military action if Assad were to again use barrel bombs filled with chlorine -- a brutal weapon the regime has deployed multiple times.
Watch: Tillerson hopes Russia will 'abandon Syria'
The White House on Monday appeared to suggest the use of chlorine barrel bombs would constitute a threshold for US action, though officials later backtracked.
Chlorine is itself not a banned substance and has broad industrial uses, though its use as a weapon is outlawed under international treaties.
"Chemical weapons are chemical weapons," Mattis said, declining to elaborate on US policy concerning chlorine.
"It's not about whether it's delivered with an artillery shell or it's delivered by a helicopter with a barrel bomb or a fighter aircraft with a bomb. It's about chemical weapons, and we've made clear where we stand on that."
The new defense chief, a retired Marine general, said Trump had determined that a military response was the best way to deter the Assad regime from conducting further chemical strikes.
US Ambassador warns that the Trump administration will take action against chemical attacks in Syria. Source: AP
"The National Security Council considered the international prohibition against the use of chemical weapons, the Syrian regime's repeated violations of that international law and the inexplicably ruthless murders the regime had committed," he said.
The Pentagon chief also emphasized Washington's focus on defeating IS extremists.
"ISIS represents a clear and present danger and immediate threat to Europe and ultimately a threat to the United States homeland," he said.
Mattis' comments came just after US Secretary of State Rex Tillerson arrived in Russia on Tuesday to confront the Kremlin over its support for Assad.
A US official speaking on condition of anonymity said Washington was investigating whether Russia was complicit in the alleged chemical weapons attack.
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