Iran, a key ally of Syrian President Bashar al-Assad, will be invited to participate in talks in Vienna to discuss ending the conflict in Syria, the US says.
The State Department said on Tuesday that it expected Damascus' key ally Iran to be invited to the talks, which will be attended by US Secretary of State John Kerry and his Russian counterpart Sergey Lavrov along with key Arab and European diplomats.
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"An invitation to Iran to participate, I think Iranian leaders can take to mean that it's a genuine multilateral invitation," department spokesman John Kirby told AFP news agency.
US officials would not say which power would pass the invitation to Tehran and did not know if Iran would accept, but they said it would be welcome to attend if it did.
Diplomatic efforts to end the civil war are stalemated, with Russia and Iran sticking with Assad while the US and Saudi Arabia insist he must agree to step down, even if not immediately.
The aim of the talks, Kirby said, is to make a start on agreeing a framework for a political transition to end the war and pave the way for Assad's exit.
"So I can't tell you exactly what the outcome of the meetings on Friday is going to be or if it's the last chapter - I rather doubt that," he said.
Iran has spent billions of dollars in the past four years to keep Assad regime in power, with hundreds of its troops on the ground.
About 2,000 Iranian troops were in Syria and Iraq helping the governments in Damascus and Baghdad, a top US military officer has said.
Marine Corps General Joseph Dunford, the chairman of the Joint Chiefs of Staff, said on Tuesday the number of Iranian forces in Iraq had fluctuated over time.
US to intensify campaign
On the military front, the Pentagon said it may launch more air strikes and even direct ground attacks by special forces against armed fighters seeking to carve out an Islamic caliphate in Iraq and Syria.
An invitation to Iran to participate, I think Iranian leaders can take to mean that it's a genuine multilateral invitation
Testifying to lawmakers, Defense Secretary Ash Carter said US forces would not shy away from "direct action on the ground" if they see a chance to hit ISIL targets.
Obama's administration has not committed ground forces to back opposition and Kurdish rebels fighting the ISIL in Syria, but has 3,500 troops in Iraq, which have a "train and advise" role.
He told senators the international coalition led by the US would focus on the ISIL stronghold of Raqqa in Syria.
"We expect to intensify our air campaign, including with additional US and coalition aircraft, to target ISIL with a higher and heavier rate of strikes," he told the Senate Armed Services Committee.
US-led efforts to "degrade and destroy" the ISIL group have been stymied by the chaos in Syria, where Alawite president Assad is battling a broader rebellion.
The US blames Assad's brutality against his own people for causing the war and preventing a united Syrian front against the ISIL group's advance.
Washington’s effort to include Iran in diplomatic process might anger Tehran's bitter foe Saudi Arabia.
The White House said that President Barack Obama and Saudi King Salman had discussed increased support for the moderate Syrian opposition in a call on Tuesday. It is unclear whether they discussed this week's meetings in Vienna.