"The Syrian government has lost the ability to refuel or rearm aircraft at Shayrat airfield and at this point, use of the runway is of idle military interest," Mattis said, describing the strike as a "measured response."
The updated assessment was released after US President Trump asked Mattis for more information on the extent of the damage to the airbase, according to a senior administration official.
The US fired 59 Tomahawk missiles at the Syrian airbase after more than 80 people were killed, including children
, in chemical weapons attack in Idlib province.
"The Syrian government would be ill-advised ever again to use chemical weapons," Mattis said.
The US blames Syrian President Bashar al-Assad for the carnage, but the Syrian military denies responsibility for the attack.
US officials originally said 58 of the 59 missiles fired Friday had "severely degraded or destroyed"
their original target, including 20 planes which they said were taken out in the strike
. On Friday, Russia's Defense Ministry said in a statement only 23 of the US missiles made it to the airbase, blowing up just six MiG-23 planes.
Military flights from the Shayrat airbase resumed over the weekend
, according to both Syrian state media outlets and an opposition group.
Defense analysts IHS Jane's estimate the Syrian airforce has about 262 strike aircraft, although the company admits this may be an imperfect number due to the ongoing war.
Confusion over US barrel bomb remarks
The Trump White House's future policy on Syria is still unclear after conflicting statements by press secretary Sean Spicer, Secretary of State Rex Tillerson and UN Ambassador Nikki Haley.
During his daily briefing, Spicer twice emphasized the US would respond to any use of barrel bombs against civilians by Assad's government.
"If you gas a baby, if you put a barrel bombing to innocent people, I think you can see a response from this president. That's unacceptable," Spicer said.
Assad's regime is accused of regularly using barrel bombs against civilians,
making Spicer's comments potentially a bold new line in the sand for the Trump administration.
But multiple White House officials quickly clarified Spicer's remarks after the briefing, implying that the press secretary had misspoken. "(Spicer) did not signal a change in administration policy," an official said.
Tillerson, Haley clash over Assad's future
Meanwhile Tillerson and Haley have made differing statements on how the US plans to deal with Syrian leader Assad.
Tillerson, who is currently in Italy for the two-day G7 foreign ministers' meeting, said Monday the US would take a stand against any government who committed atrocities on innocent people.
"We will rededicate ourselves to holding to account any and all who commit crimes against innocents anywhere in the world," he said, speaking at an Italian war memorial.
But he has stopped short of calling for Assad's removal from office. During an interview on CBS Sunday, he said destroying ISIS was Trump's top priority.
"I think the President has been quite clear. First and foremost, we must defeat ISIS," he said, when asked about the possibility of deposing Assad.
In comparison, Haley told CNN Sunday that removing Assad was a US priority and inevitable. "If you look at his actions, if you look at the situation, it's going to be hard to see a government that's peaceful and stable with Assad," she said.