An air strike by the US airforce in response to a suspected chemical attack damaged or destroyed 20 percent of Syria's operational aircraft, US defence secretary James Mattis says.
Mr Mattis said Syria would be "ill-advised ever again to use chemical weapons".
Syria has denied the chemical attack on the rebel-held town of Khan Sheikhoun last week that left 89 people dead.
G7 nations are meeting in Italy to discuss policy and how to persuade Russia to abandon its Syrian ally.
The US fired 59 cruise missiles at Syria's Shayrat airbase on Thursday, following the suspected chemical attack a day before.
Mr Mattis said the "measured response" by the US had "resulted in the damage or destruction of fuel and ammunition sites, air defence capabilities and 20 percent of Syria's operational aircraft".
"The Syrian government has lost the ability to refuel or rearm aircraft at Shayrat airfield."
The Syrian military admitted significant material damage but a Russian defence ministry spokesman said only six Syrian Air Force MiG-23s, plus a number of buildings, were destroyed and that only 23 of the missiles had reached Shayrat.
Mr Mattis said the strike had shown the US would "not passively stand by while [Syrian President Bashar al-Assad] murders innocent people with chemical weapons".
White House spokesman Sean Spicer said further strikes were on the table.
"The sight of people being gassed and blown away by barrel bombs ensures that if we see this kind of action again, we hold open the possibility of future action," he said.
What is being discussed at the G7?
The two-day meeting of foreign ministers in Lucca in Tuscany aims to hammer out a unified approach to the Syria conflict.
UK Foreign Secretary Boris Johnson said the message from the meeting should be clear - that Russian President Vladimir Putin must be made to abandon his support for Mr Assad.
"He's toxifying the reputation of Russia by his continual association with a guy who has flagrantly poisoned his own people," Mr Johnson said.
He said further sanctions on Russia would also be discussed.
Russia is already under a raft of sanctions imposed by the US and EU in response to the annexation of Crimea and the crisis in eastern Ukraine.
These would be the first sanctions against Russian figures over Syria if they were to be adopted, but it is far from clear they will be.
Russia said the US had failed to provide evidence that Syria has chemical weapons.
Russia and Iran, President Assad's key military backers, have also threatened retaliation if there were any further US air strikes.