Russia and Iran are criticizing the United Nations for withdrawing its invitation to Iran to join the Syrian peace talks this week.
U.N. Secretary General Ban Ki-Moon reversed course after Iranian leaders refused to agree the talks are intended to create a transitional Syrian government with full executive powers, as decided during the first round of talks in 2012 in Geneva.
The last-minute invitation to Iran appeared to undermine the conference. Western-backed Syrian rebels threatened to drop out altogether unless the invitation was withdrawn or Iran agreed to the terms set forth during the first round of talks.
Syrian rebels also said Iran must agree to withdraw its "troops" and Hezbollah fighters from Syria before an Iranian delegation could attend the second Geneva peace talks.
But Russia and Iran, Syrian President Bashar Assad's two closest allies, criticized the United Nation's decision to exclude Tehran.
In Moscow, Russian Foreign Minister Sergey Lavrov called Ban's decision a mistake, saying Moscow would try to make the negotiations work.
The controversy over Iran's participation reflected the deep differences between the United States and Russia on Syria.
Russia has been a key ally of Damascus, shielding Assad's regime from U.N. sanctions and supplying it with weapons throughout the civil war that has killed more than 130,000.