U.S. Secretary of State John Kerry urged members of the Syrian opposition to decide in favor of attending next week's peace talks in Switzerland when they vote on the question on Friday.
Earlier, the National Coordination Body, a Syrian opposition group that some rebels see as a front for President Bashar al-Assad, has decided not to take part in peace talks that will begin next week, international mediator Lakhdar Brahimi said on Thursday.
Brahimi said in a statement that he respected the organization's decision not to join the opposition delegation to the talks but deeply regretted they would not be included. Brahimi has yet to announce the make-up of the two delegations to the talks, which are set to begin on Jan 22.
The National Coordination Body is also known as the National Coordination Committee.
This comes after Foreign Minister Mohammad Javad Zarif said that Iran wants to take part in next week's international peace conference on Syria but will not attend if conditions are set for its participation.
The United States says Iranian delegates should be at the conference in Montreux, Switzerland, only if they are willing to accept an agreement reached at a June 2012 peace conference in Geneva calling for a transitional government body to be established in Damascus "by mutual consent".
Washington interprets that language as requiring President Bashar al-Assad's departure from power but Russia and Iran reject this interpretation.
"If the Islamic Republic of Iran is invited like all other participants in the conference, it will take part in this event," Zarif told a news conference after talks with Russian Foreign Minister Sergei Lavrov.
Lavrov urged U.N. Secretary-General Ban Ki-moon to invite Iran and Saudi Arabia to the conference starting on Jan. 22, saying the participation of regional powers would help find a political solution to the nearly three-year-old conflict.
More than 1,000 killed in Syria rebel infighting:
More than 1,000 people have died in two weeks of fighting between al Qaeda-linked militants in Syria and rival rebels, a monitoring group said on Thursday, by far the bloodiest internecine violence since the revolt began.
The Britain-based Syrian Observatory for Human Rights, a pro-opposition monitoring group, said 1,069 people had been killed in clashes and executions since fighting between the al Qaeda-linked Islamic State of Iraq and the Levant (ISIL) and rival rebel forces erupted at the start of this month.
The count included 130 civilians, 21 of them executed by ISIL at the children's hospital that was the group's base in the northern city of Aleppo, the Observatory said.
Another 608 of them were fighters from the Islamist rebel groups and other less ideological rebel groups that have joined forces to confront ISIL.
Fighters from the al Qaeda affiliate accounted for 312 of the dead and the remaining 19 were unidentified, the Observatory said.
UN: Mass 'executions' in Syria could be war crime
The United Nations human rights office said a "soaring" number of executions in northern Syria could amount to war crimes.
In a statement, U.N. High Commissioner for Human Rights Navi Pillay warned armed opposition groups that such executions violate international humanitarian law.
Pillay said Thursday that over the past two weeks her office has received reports of "a succession of mass executions of civilians and fighters who were no longer participating in hostilities in Aleppo, Idlib and Raqqa by hardline armed opposition groups in Syria, in particular by the" al-Qaeda linked Islamic State of Iraq and Syria.
Pillay said exact numbers are hard to verify but that "many civilians and fighters in the custody of extremist armed opposition groups have been executed since the beginning of this year."
UK to give Veolia contract to destroy chemicals from Syrian arms arsenal
Britain is to award a contract to destroy around 150 tonnes of chemicals from Syria's chemical weapons program to Veolia Environment, two sources familiar with the matter told Reuters on Thursday.
The chemicals will be processed at the French water and waste management firm's incineration plant at Ellesmere Port in Cheshire, England, the sources said, saying an announcement was due later on Thursday.
This comes as the head of the U.N. chemicals watchdog said that the removal and destruction of the most dangerous agents in Syria's chemical arsenal may not be completed until the end of June because of logistical and security problems.
The destruction of "primary" chemicals - mustard gas and the components for making Sarin and VX - had originally been foreseen completed by the end of March, but the civil war, bad weather and bureaucracy have slowed it.
Nonetheless, Ahmet Uzumcu, head of the Organisation for the Prohibition of Chemical Weapons, told reporters in Rome he was confident that all the chemicals could be destroyed by the end of June, the deadline set for only the destruction the less dangerous "commodity" chemicals.
Uzumcu said that more than 16 tonnes of the total of 560 tonnes of the "primary" chemicals had so far been shifted to a Danish vessel in the Syrian port Latakia, conceding: "The quantity of the first movement is not that high."
The ship is now waiting in international waters offshore for more shipments.
Uzumcu is in Italy to address parliament about the transfer of the primary agents at an Italian port from the Danish vessel to an American one, where they will be destroyed at sea. The Calabrian port of Gioia Tauro has been chosen for the chemical transfer, an Italian government source said.
AP/REUTERS