Washington is in talks with the UN over the invitation of Iran to Syria peace talks and expects the invite to be withdrawn,

Washington is in talks with the UN over the invitation of Iran to Syria peace talks and expects the invite to be withdrawn, a US official said Monday.

Iran has "never endorsed the Geneva-I communiqué" which calls for a transitional government in Syria and "we expect the invitation will be rescinded," a senior State Department official said.

Syrian peace talks, due to open on Wednesday in the Swiss lakeside city of Montreux, were in disarray even before they began after the UN invite to Iran sparked a boycott call from the opposition.

Another senior US official, in Istanbul meeting with the Syrian opposition, confirmed that they had suspended their agreement to join the talks.

He accused Iran of not working for an end to the fighting. They are "escalating tensions on the ground" by mobilizing foreign militias to fight for Assad and providing "substantial military" support, he told reporters on a conference call.