The Obama administration joined U.N. diplomats Wednesday in condemning yet another massacre of Syrian civilians, while also acknowledging deepening pessimism for ending the violence as prospects for a diplomatic solution appeared blocked both at the United Nations and in Syria.

U.N. cease-fire monitors reported observing the bodies of 13 electrical workers — all of whom had had their hands bound and had been shot execution-style — in Syria’s Deir al-Zour province, a grisly find that came just four days after the discovery of a mass slaughter of 108 civilians, including women and children, in the Houla region, near Homs.

In New York and Washington, administration officials and diplomats warned that the Syrian conflict appeared to be entering a more dangerous phase, and they vented frustration over the continuing violence.

“We may be seeing the wheels coming off the bus,” Susan E. Rice, the U.S. ambassador to the United Nations, told reporters after diplomats had been briefed by U.N. officials in private on the latest findings by almost 300 U.N. observers posted through Syria.

Echoing remarks made this week by Kofi Annan, the U.N. special envoy to Syria, Rice said the 14-month conflict had “in effect reached the tipping point.”

But even as they expressed revulsion over the latest killings, their efforts to force an end to the crisis appeared to be deadlocked. Several senior U.S. officials, in separate comments, acknowledged that the U.N. deployment of monitors had failed to deter government attacks on civilians.

“It is our belief — and evidence of this is mounting — that putting monitors into the country is simply not going to stop the violence,” Denis McDonough, President Obama’s deputy national security adviser, said in a policy speech Wednesday to the U.S.-Islamic World Forum in Doha, Qatar. “There needs to be a credible transition process that gets underway.”

But officials also acknowledged the dearth of palatable options for dislodging Syrian President Bashar al-Assad. Russia, which has blocked proposed U.N. resolutions intended to increase pressure on the Syrian government, said it would continue to resist efforts to impose new sanctions.

“Our attitude to sanctions, frankly, continues to be negative,” Russia’s U.N. envoy, Vitaly I. Churkin, told reporters. He rebuked unnamed governments for continuing to encourage the resistance through the supply of weapons, noting that the armed opposition are “better and better equipped.”

Syria’s U.N. ambassador, Bashar al-Jaafari, denied that his government was involved in the weekend massacre, blaming “armed terrorists” seeking to foment sectarian violence, and claiming that 26 Syrian security officers were killed battling them.

Jaafari said that the Syrian government had established a national commission to investigate the violence and that the panel would announce its conclusion as early as Thursday.

Lynch reported from the United Nations.