(Adds U.S. quotes, background)
By Lesley Wroughton
WASHINGTON, Jan 20 (Reuters) - The United States insisted on Monday that a U.N. invitation to Iran to attend a Jan. 22 peace conference on ending Syria's war should be withdrawn unless Tehran fully supports a 2012 plan to establish a transitional government in Syria.
A senior State Department official, speaking on condition of anonymity, said chances of the conference going ahead were still "fluid" given that Iran has not fully endorsed the Geneva 1 agreement from 2012 to end the conflict.
The 2012 Geneva 1 plan agreed to establish "by mutual consent" a transitional body to govern Syria.
Syrian opposition groups, which voted on Saturday to attend the conference, have threatened to withdraw from the talks unless the invitation to Iran is withdrawn.
The official said Iran was providing substantial military and economic support for President Bashar al-Assad and Tehran's participation in peace talks was not helpful.
"They are doing nothing to de-escalate tensions ... and their actions have actually aggravated them, and so the idea that they would come to the conference refusing to acknowledge support for Geneva 1, we do not see how it could be helpful," the official said.
Another U.S. administration official said Secretary of State John Kerry was in talks with U.N. Secretary-General Ban Ki-moon over the Iran standoff, two days before the planned talks between the Syrian government and rebel groups.
"Iran has not publicly endorsed Geneva, not before last night and not overnight. We expect the invitation will be rescinded," the official added. "We have been in close touch with the U.N. and will see what happens over the course of the day."
"(Iran) has not met the conditions that the United States has set out," the State Department official said, adding, "They do not meet the bar."
Iran rebuffed on Monday a precondition for taking part in the peace talks this week, saying it could not accept a plan for a Syrian political transition agreed in 2012, according to the ISNA news agency.
The administration official said statements by Iran since the invitation was issued on Sunday "falls short of what the U.N. was told privately" by Iranian officials.
Ban said on Sunday Iran had privately assured him they would play a "positive and constructive role" in the peace talks. Some 30 countries have been invited to attend the talks, which are unlikely to lead to an immediate political settlement.
Kerry has said the talks should be seen to be the start of a process and emphasized that there is no place for Assad in Syria's future. (Reporting by Lesley Wroughton; Editing by Chizu Nomiyama and Stephen Powell)