The UN failed to break a deadlock at Syrian peace talks in Geneva on Tuesday, with negotiations interrupted after President Bashar al-Assad's regime pushed for a statement condemning the United States.
After announcing the start of a fourth day of talks with a morning session around 11:00 am (1000 GMT), the UN said in a terse statement later that "no meeting has been planned for this afternoon".
A member of the opposition negotiating team, Rima Fleihan, told AFP that UN mediator Lakhdar Brahimi had adjourned the meeting "because the regime is not cooperating on any subject, not on humanitarian issues and not on a transitional governing body."
She said the opposition had presented a preliminary transition plan laying out its "vision" for Syria, but the regime refused to engage in talks.
"We have a vision, unfortunately the regime presented nothing and refused all discussion," Fleihan said, adding that the next session was planned for Wednesday morning.
Tuesday morning's session saw regime delegates present a statement condemning Washington, which it wanted adopted by participants.
The statement, obtained by AFP, said "the United States has made a decision to resume arming terrorist groups in Syria."
"This decision can only be understood as a direct attempt to obstruct any political solution in Syria through dialogue," it said.
It follows a report from the Reuters news agency that the US Congress secretly approved funding for weapons deliveries to "moderate" Syrian rebel factions.
Syrian Deputy Foreign Minister Faisal Muqdad said that showed Washington "is not interested in the success" of the peace process.
He accused the opposition of obstructing talks by refusing to adopt the regime statement.
"The other party said they don't agree... and that they support the United States' steps to arm terrorist groups," he said.
Delegates from the regime and the opposition National Coalition have been brought together in the biggest diplomatic push yet to end a civil war that has left more than 130,000 dead and forced millions from their homes.
The talks hit an impasse on Monday when discussions started on political issues, including the transfer of power to a transitional government.
Brahimi had said Tuesday's talks would focus on the Geneva communique, the text agreed by world powers in 2012 that calls for the creation of a transitional governing body in Syria.
He said he also hoped for concrete steps on humanitarian aid, especially for besieged and starving families in rebel-held areas in the central city of Homs.
Brahimi admitted Monday that the talks so far "haven't produced much", but said just getting the two sides to sit in the same room was a step forward.
Expectations are low for a breakthrough, especially after the two sides failed to agree on even the basic principles of political talks on Monday.
Despite their frustration, each side vowed it would not be the first to walk away from the talks, which are expected to last until Friday.
In the only tangible promise to emerge from the meetings so far, Brahimi said Sunday the regime had agreed to allow women and children safe passage from besieged rebel-held areas of Homs.
But there has been no movement since, on either an evacuation or opposition demands that aid convoys be allowed in the areas.
The Old City of Homs has been under siege since June 2012 after rebels there rose against the regime, with an estimated 500 families living with near-daily shelling and the barest of supplies.
UN bodies and the International Committee of the Red Cross have said they are on standby with aid but are waiting for approval to move in.
Activists in Homs on Tuesday urged opposition figures at the talks to push for the lifting of the 600-day siege.
"We need the siege lifted and to ensure that residents can enter and exit through safe corridors, without passing through regime checkpoints," the activists said in a statement.
Should the siege remain in place, "all solutions will be futile, and will do nothing to end this tragedy," they said.