Russia and the United States headed for a clash at the UN Security Council on Friday over a meeting called by Washington to discuss the wave of deadly protests in Iran.
US Ambassador Nikki Haley has warned the demonstrations could escalate into full-blown conflict, drawing a comparison with Syria.
But Russia accuses the United States of interfering in Iran's national affairs and called for closed-door talks just before the council's meeting, scheduled for 3:00 pm (2000 GMT).
A total of 21 people have died and hundreds have been arrested since December 28 as protests over economic woes turned against the Iranian regime, with attacks on government buildings and police stations.
"The world has witnessed the horrors that have taken place in Syria, that began with a murderous regime denying its people's right to peacefully protest," Haley said in a statement late Thursday.
"We must not let that happen in Iran."
On Thursday, the United States formally requested an open meeting, and Russia then called for the closed-door talks at 2:30 pm (1930 GMT) Friday.
Diplomats expect Russia to try to block the formal council meeting by requesting a procedural vote to decide whether the situation in Iran should be on the agenda.
For a new agenda item to be discussed at the Security Council, at least nine of the 15 council members must support holding the meeting. No vetoes apply.
"This is a matter of fundamental human rights for the Iranian people, but it is also a matter of international peace and security," said Haley.
"It will be telling if any country tries to deny the Security Council from even having this discussion, just as the Iranian regime tries to deny its own people the ability to have their voices heard."
- Russia warns US -
In Moscow, Russia's Deputy Foreign Minister Sergei Ryabkov made it clear that the protests in Iran did not pose an international threat and should not be a topic for debate within the council.
"The United States continues to interfere both openly and covertly in the internal affairs of other countries. They do so shamelessly," Ryabkov was quoted as saying by Interfax news agency.
"This is how we view the American initiative to convene the UN Security Council over a situation that is of purely national concern in Iran."
Ryabkov accused Washington of "directly attacking the sovereignty of other states under the pretext of being concerned about democracy and human rights."
Iran's UN Ambassador Gholamali Khoshroo on Wednesday sent a letter to the council accusing the United States of meddling after US President Donald Trump pledged to help Iranians "take back" their government.
Pro-regime rallies were held in Tehran after Friday prayers, the third straight day of marches in support of the government, which has declared the unrest over.
Over the past days, the United States has lobbied hard to win support for the meeting, especially from the six new non-permanent council members, diplomats said.
Equatorial Guinea, Ivory Coast, Kuwait, Peru, Poland and the Netherlands began their stint at the Security Council on January 1.
If the meeting goes ahead, the council is not expected to issue a statement on the unrest in Iran, which would have to be agreed by all 15 members, diplomats said.
The US administration has also imposed unilateral sanctions on five Iranian companies linked to Tehran's ballistic missile program.