The absence of key opposition groups is threatening to derail the long-anticipated start of Syrian peace talks in Geneva on January 29.
A coalition of opposition groups backed by Saudi Arabia said it would not attend the negotiations until an agreement is reached with the Syrian government on providing aid to besieged towns.
Also missing from the talks were representatives of Syria's Kurdish population, which has control over much of the country's north. The Kurds were not invited because another key participant, Turkey, insists their representatives are linked to an outlawed Kurdish party in Turkey.
The U.S. State Department on January 28 pleaded with the Saudi-backed coalition, known as the High Negotiations Committee, to join the talks despite seeing no progress on bringing aid to besieged and starving Syrians.
"These demands, while legitimate, shouldn't keep the talks from moving forward," said department spokesman Mark Toner. "This is really an historic opportunity for them to go to Geneva to propose serious, practical ways to implement a cease-fire and other confidence-building measures."
The United Nations envoy for Syria, Staffan de Mistura, who organized the talks, made a last-minute plea directly to Syrians in a video, urging them to support the talks as what he portrayed as a last chance for peace that "cannot fail."