They said the Syrian government would only discuss political transition if all sides agree to a joint fight on terrorism.
The remarks -- which the New York Times said are unlikely to produce compromise because the Syrian government defines all its armed opponents as terrorists -- added to an increasingly grim mood in peace negotiations.
The statements came a day after Russian, U.S. and U.N. officials failed to reach consensus on pushing the parties toward meaningful negotiations. A week of meetings has not even produced an agenda for discussion, the Times noted.
U.N. mediator Lakdar Brahimi said Thursday the Syrian government walk away from the negotiating table, leaving a 24-point plan, presented by the opposition for structuring a political transition for Syria, unread and untouched on the table.
"All they (the opposition representatives) want to talk about is creating a transitional governing body," said Russian Foreign Minister Sergei Lavrov.
The U.N. Security Council Thursday considered two resolutions on humanitarian access in areas of Syria ravaged by the nearly 3-year-old civil war, while the U.N. mediator for Syria talks warned peace talks between the government and opposition representatives in Switzerland were in danger of collapsing, the Times reported.
While delivery of humanitarian aid to the besieged Syrian city of Homs is remarkable, it cannot serve as a model elsewhere, U.N. official Valerie Amos said.
"The cease-fire agreement which enabled us to finally access the Old City [neighborhood] of Homs shows what can be done when parties to this conflict put their differences aside in support of humanitarian action," Amos said Thursday after briefing the U.N. Security Council on Syria.
"While remarkable, the events in the Old City cannot serve as a model," Amos, the organization's top official for emergency operations, said in a release. "Why? It was a success, given the difficult circumstances, but I find it difficult to describe as progress."
In the Security Council meeting Thursday, Australia, Jordan and Luxembourg proposed one resolution that called for punitive measures on those obstructing aid delivery and named specific besieged communities in need of aid. Russia, which had dismissed the resolution as a "non-starter," proposed its own enforcement language and did not list the besieged communities, the Times said.
In Geneva, Lakhdar Brahimi, the U.N. mediator, told senior U.S. and Russian officials the Syrian government has refused to compromise even on the agenda, two Western diplomats told the Times.
One diplomat said Brahimi was "very blunt" and indicated he might call off the talks rather than preside over discussions that produce nothing of substance, particularly if Russia -- Syria's main ally -- won't intervene.
In the four months since the Security Council adopted a statement stressing the need to protect civilians and give access to people in need, the fighting has intensified, Amos said.
"All parties are failing in their responsibility to protect civilians," Amos said. "We understand that a war is going on. But even wars have rules."
A local cease-fire allowed U.N. convoys to deliver a month's worth of food to people trapped by a two-year government blockade of Homs and to evacuate 1,400 people.
Amos outlined what she called "modest progress" in delivering humanitarian aid to Syrians since October:
-- Twenty-six inter-agency convoys have delivered one-off assistance in hard-to-reach areas in Aleppo, Ar-Raqqa, Dara'a, Hama, Homs, Idleb, and rural Damascus since the statement was adopted.
-- Since Jan. 18, the U.N. Relief and Works Agency has delivered food enough for two weeks to more 6,500 families and vaccines for 10,000 children.
-- Last week food and medical supplies for 2,500 people in the Old Homs neighborhood was delivered and more than 1,400 people were evacuated.
During a sideline meeting in Geneva with U.S. and Russian representatives, Brahimi told Russian Deputy Foreign Minister Gennady Gatilov the Syrian regime rejected an opposition overture that would allow simultaneous discussion of the opposition-required transitional government without President Bashar Assad and the regime demand that talks focus on combating terrorism, a Western diplomat told the Wall Street Journal.
Hoping for Russian intervention, Brahimi asked Gatilov "point blank" if Moscow could consider supporting discussing a political transition in parallel with terrorism, a person knowledgeable about the talks said.
"No," Gatilov said, the person recounted.
"Failure is always staring us in the face," Brahimi said after the closed-door session with Gatilov and U.S. Undersecretary of State Wendy Sherman.
"As far as the United Nations is concerned, we will certainly not leave one stone unturned if there is a possibility to move forward," he said.