Spokesman Rear Adm John Kirby said fighter and bomber jets and Tomahawk missiles were used in the attack.
The strikes were expected as part of President Barack Obama`s pledge to "degrade and destroy" IS, which has taken huge swathes of Syria and Iraq.
The US has already undertaken 190 air strikes in Iraq since August.
But Monday`s action expands the campaign against the militant group over the national border into Syria.
The group has taken control of a vast area between Syria and Iraq, executed captive soldiers, aide workers and journalists, and threatened the mass killing of religious minorities in Iraq.
On Monday, Rear Adm Kirby confirmed "US military and partner nation forces" were undertaking military action in Syria but did not give details.
"Given that these operations are ongoing, we are not in a position to provide additional details at this time," he said in a statement.
The decision to strike in Syria was made by the head of US Central Command Gen Lloyd Austin, Rear Adm Kirby said, "under authorisation granted him by the commander in chief".
The BBC has learned that Arab nations were among those involved in the strikes in Syria.
In a nationally televised speech outlining his strategy against IS earlier this month, Mr Obama said that any group that threatened America would "find no safe haven", including inside Syria.
Mr Obama has said the strikes were within his power as commander in chief but has asked Congress to authorise a separate mission to train and arm Syrian rebels fighting against IS.