The US State Department announced on Tuesday that Iran would be asked to take part in talks on resolving Syria's civil war to be held in Vienna later this week. A staunch ally of Syrian President Bashar al-Assad, Tehran's participation risks angering another key member of the talks - arch-foe Saudi Arabia.

"An invitation to Iran to participate, I think Iranian leaders can take to mean that it's a genuine multilateral invitation," said department spokesman John Kirby, without saying which power would officially pass the invitation.

Separately, President Barack Obama called Saudi King Salman on Tuesday, but did not confirm if it was to reassure the monarch about Iran's role in the talks.

Kirby told the press that the aim of the Vienna talks was to lay plans for an interim unity regime in Damascus that would pave the way for Assad's eventual exit.

Carter alludes to possible use of US ground troops

At the same time, Secretary of Defense Ash Carter testified before Congress on the US military strategy against "Islamic State" (IS) terrorists in Iraq and Syria. Signaling a shift in policy, Carter also said that the Obama administration would not shy away from "direct action on the ground" by special forces if it afforded a better chance to strike IS targets.

Carter also told the Senate Armed Services Committee that the military intends to step its campaign in the region.

"We expected to intensify our air campaign, including with additional US and coalition aircraft, to target ISIL with a higher and heavier rate of strikes," Cater said, using an alternative acronym for IS. He went on to described a strategy centered on Raqqa and Ramadi, the latter of which is the capital of Iraq's central Anbar province and has been under IS control since May.

"We are willing to continue providing more enabling capabilities to help our Iraqi partners succeed," Carter told the committee.

The US currently has 3,500 ground troops stationed in Iraq, who despite their stated mission to "train and advise" Iraqi military, have been recently drawn back into intense fighting in the country's north. Last week, a US soldier died when American commandos and Kurdish peshmerga forces stormed an IS-run jail and freed 70 captives.

es/bw (AFP, dpa)