Iran has yet to respond to the invitation, which was extended by Russia. The next round of talks will take place Thursday in Vienna.
The United States had previously considered Iran joining the talks, but the invitation was finally given after days of behind-the-scenes negotiation, particularly with Saudi Arabia, Iran's regional rival.
Iran's arrival at the table could be a gamble for the United States, which has called for the removal of Syrian President Bashar Assad. Tehran has backed Assad throughout the conflict, even fighting alongside his forces.
On the other hand, all multilateral efforts to date have failed to curb the bloodshed.
The United States remains committed to finding a solution that includes the removing Assad from power, according to State Department spokesman John Kirby.
"The ultimate goal that everyone wants to get to … is to come up with a framework for a successful political transition in Syria which leads to a government not led by Bashar al-Assad and that is representative of and responsive to the Syrian people," Kirby said on Tuesday.
Russia and Iran reject US insistence on Assad's removal, with Moscow stressing that only the Syrian people can select their country's leadership.
The United States, Russia, Saudi Arabia and Turkey met last week in Vienna. This week's gathering will expand to include countries such as Britain, France, Germany, Jordan, the United Arab Emirates, and now, possibly, Iran.
The war in Syria has killed at least 250,000 people and forced more than 11 million from their homes, the Associated Press reported.