The US central command said the strikes were launched on Tuesday and were ongoing. It did not provide further information about the locations of the attacks, although IS's main stronghold is in Raqqa province.
"I can confirm that US military and partner nation forces are undertaking military action against IS terrorists in Syria using a mix of fighter, bomber and Tomahawk land attack missiles," a spokesman for the department of defence said.
"The decision to conduct theses strikes was made earlier today by the US central command commander under authorisation granted him by the commander in chief. We will provide more details later as operationally appropriate."
Al Jazeera's Sue Turton, speaking from across the border in Iraq, said the attacks in Syria appeared to be part of a broader campaign than that undertaken in northern Iraq. It comes weeks after the US formed a coalition to confront the IS group, which has taken over large areas of Syria and Iraq and declared a "caliphate". It includes Nato, Australia and Arab nations.
The US statement on the attacks in Syria did not say which other nations were involved. An unnamed US official told the Reuters news agency that Arab partners were "helping", although the role was not fully explained. France launched its first attacks on IS in Iraq last week but gave no indication it would expand its campaign to Syria.
The US president, Barack Obama, said on September 10 that the expanded campaign would degrade and ultimately destroy the group.
The attacks come a day after IS's spokesman, Abu Mohammed al-Adnani, called on supporters of the group to attack foreigners wherever they are.
In a 43-minute video, Adnani said: "If you can kill a disbelieving American or European - especially the spiteful and filthy French - or an Australian, or a Canadian, or any other disbeliever from the disbelievers waging war, including the citizens of the countries that joined a coalition against the Islamic State, then rely upon God, and kill him in any manner.''