Syrian President Bashar al-Assad declared on Friday that he would not negotiate with armed groups, appearing to scupper peace talks that Russia and the United States hope to bring about next month.
Washington helped broker an agreement reached on Thursday by more than 100 members of Syria's opposition parties and more than a dozen rebel fighting groups ranging from Islamists to Western-backed Free Syrian Army (FSA) groups - but not Islamic State - to send a joint team to meet the government under U.N. auspices next month.
The initiative is driven at least partly by their focus on defeating a common enemy in the form of Islamic State, which has seized large parts of Syria and Iraq and is increasingly ordering or inspiring attacks on the West and Russia.
But in an interview with the Spanish news agency EFE, Assad said he would not hold political talks with any armed groups, and accused Washington and its ally Saudi Arabia of wanting "terrorist groups" to join negotiations.
He said Syria had had contact with armed groups for one reason only: "to reach a situation where they give up their armaments and either join the government or go back to their normal life ... This is the only way to deal with the militants
"There's no point in meeting in New York or anywhere else without defining terrorist groups," he said. "For us, in Syria, everyone who holds a machinegun is a terrorist."
Foreign ministers of countries opposed to Assad are due to meet in Paris on Monday to prepare for talks with Russia and Middle Eastern countries in New York on Thursday with a specific focus on trying to form the opposition delegation for the peace talks.
Before Assad's remarks were published, Washington said U.S. Secretary of State John Kerry would travel to Moscow on Tuesday for talks with President Vladimir Putin and Foreign Minister Sergei Lavrov.
The experience of a failed peace conference in Switzerland two years ago had kept expectations for the latest push low.
Demands that Assad leave
The rebel groups issued a statement at the end of their two-day conference in Riyadh saying Assad should leave power at the start of a transitional period, and calling for an
all-inclusive, democratic civic state.
Although the demand goes beyond what Western powers are calling for, it is rejected by Assad's patrons Russia and Iran.
At one point on Friday, Putin appeared to hold out an olive branch to the West, making what appeared to be Russia's first explicit statement of support for rebels opposed to Assad in the fight against Islamic State.
At an annual meeting at the Defence Ministry, he said the Free Syrian Army was engaged in "offensive actions against terrorists, alongside regular forces, in the provinces of Homs, Hama, Aleppo and Raqqa".
"We support it from the air, as well as the Syrian army, we assist them with weapons, ammunition and provide material support." he said.
But FSA groups dismissed any suggestion of Russian support.
Russian air strikes have targeted a number of FSA groups in western Syria, notably factions that have received military support from Assad's foreign enemies, including anti-tank missiles. A senior Western diplomat said only about 20 percent of Russian strikes had been aimed at Islamic State.
A few hours later, Kremlin spokesman Dmitry Peskov declined to repeat Putin's assertion, saying: "Russia supplies weapons to the legitimate authorities of the Syrian Arab Republic."
Asked whether Putin had been talking about supplying FSA groups, he added: "Please do not cling to meanings in this case. Such an interpretation is possible."
Meanwhile, Islamic State claimed a triple truck bomb attack that killed dozens of people in a part of northeastern Syria where Kurdish YPG forces have been pushing back the Islamist militants in recent weeks.
The YPG has been the most effective partner on the ground in Syria for the U.S.-led coalition that is pounding Islamic State from the air.
But it operates only in northern Syria, and is distrusted by other rebel groups, which shut it out of the Riyadh conference along with the al Qaeda-linked Nusra Front, fighting alongside other rebels against Assad in western Syria.
Differences also remain between the groups that did attend, with many fighters considering Damascus-based political opposition figures to be too close to Assad - the same reason the YPG was excluded.
The United Nations says the war has left 13.5 million Syrians in need of help and protection, including 400,000 living under siege and 4.5 million in areas that are hard for humanitarian assistance to reach.