Iran is likely to be invited for the first time to international talks with the US and Russia over the Syrian conflict, the US says.
US state department spokesman John Kirby said it was unclear if Iran's leaders would attend the talks in Vienna later this week.
Top envoys from the US, Russia, Arab and European nations are taking part.
Syria's Western-backed opposition and the US's Gulf Arab allies have long opposed Iran's role in the Syrian war.
Mr Kirby told reporters on Tuesday that he expected Iran "to be invited to participate" in the talks in Vienna.
"Whether they come or not is up to Iranian leaders," he said. "It's important for us that key partners are in these discussions... They [Iran] could be a key partner, but they are not now."
Separately, the US said it was prepared to intensify its campaign against IS, which is also known by the acronym ISIL. The US has been carrying out air strikes against the group in Syria and Iraq since last year.
"We won't hold back from supporting capable partners in opportunistic attacks against ISIL or conducting such missions directly, whether by strikes from the air or direct action on the ground," said US Defense Secretary Ashton Carter.
Iran is Syrian President Bashar al-Assad's closest ally. It is believed to have spent billions of dollars over the past four years to prop up his government, providing military advisers and subsidised weapons, as well as lines of credit and oil.
It is also thought to have been influential in the decision of Lebanon's Hezbollah movement to send fighters to Syria to assist pro-Assad forces.
Syria crisis: Where key countries stand
Iran's government has proposed a peaceful transition in Syria that would culminate in free, multi-party elections, but it has not been involved in multilateral peace talks.
Russia, which has also said only a political solution can end the Syrian conflict, last month began an air campaign to bolster Mr Assad. It says it is targeting only "terrorists", but many strikes have hit the positions of rebel groups backed by the US and its Gulf allies.
Why is there a war in Syria?
Anti-government protests developed into a civil war that, four years on, has ground to a stalemate, with the Assad government, Islamic State, an array of Syrian rebels and Kurdish fighters all holding territory.
Who is fighting whom?
Government forces concentrated in Damascus and the centre and west of Syria are fighting the jihadists of Islamic State and al-Nusra Front, as well as less numerous so-called "moderate" rebel groups, who are strongest in the north and east. These groups are also battling each other.
What's the human cost?
More than 250,000 Syrians have been killed and a million injured. Some 11 million others have been forced from their homes, of whom four million have fled abroad - including growing numbers who are making the dangerous journey to Europe.
How has the world reacted?
Iran, Russia and Lebanon's Hezbollah movement are propping up the Alawite-led Assad government, while Turkey, Saudi Arabia and Qatar back the more moderate Sunni-dominated opposition, along with the US, UK and France. Hezbollah and Iran are believed to have troops and officers on the ground, while a Western-led coalition and Russia are carrying out air strikes.