The United States has warned Russia to rein in its ally Syria after reports that government forces have stepped up attacks on rebels in an area of southwest Syria that was designated as a "de-escalation zone" last year.
A war monitor and rebel officials said that Syrian government helicopters dropped barrel bombs on opposition areas in the zone on June 22 in defiance of repeated U.S. warnings about the need to enforce an agreement between Moscow and Washington establishing a cease-fire in the area.
U.S. Ambassador to the United Nations Nikki Haley said that the Syrian military escalation "unambiguously violates" the de-escalation arrangement and that the bombardments have already prompted more than 11,000 civilians to flee the area.
"The Syrian regime's violations of the cease-fire in southwest Syria need to stop," Haley said.
"We expect Russia to do its part to respect and enforce the cease-fire it helped establish, and to use the influence it has to stop the Syrian regime's violations and any further destabilizing actions in the southwest and throughout Syria."
"Russia will ultimately bear responsibility for any further escalations in Syria," Haley said.
Syrian President Bashar al-Assad has vowed to recapture the area bordering Jordan and the Israeli-occupied Golan Heights, and the army began ramping up its assault there this week.
Assad's campaign drew a warning from the United States earlier this week that Syria and Russia face "serious repercussions" if the cease-fire violations continue. Washingon has accused Damascus of initiating air strikes, artillery, and rocket attacks in the area.
The escalation of tensions prompted UN Secretary-General Antonio Guterres late on June 22 to say he was "gravely concerned" about the situation and call for "an immediate end" to the violence and return to honoring the cease-fire.
The Syrian Observatory for Human Rights, a Britain-based war monitor, said Syrian government helicopters on June 22 dropped more than 12 barrel bombs on a cluster of rebel-held towns, including Busra al-Harir, northeast of Deraa city, causing damage but no deaths.
Abu Bakr al-Hassan, spokesman for the rebel group Jaish al-Thawra, which fights under the banner of the Free Syrian Army, said the munitions had been dropped on three towns and villages.
Hassan told Reuters that he believes the crude bombardment is "testing" the "U.S. commitment to the de-escalation agreement in the south."
The Syrian government has denied using barrel bombs, which are containers filled with explosive material that are dropped from helicopters and which cannot be accurately aimed.
However, United Nations investigators have extensively documented the government's use of them during the seven-year civil war which has killed more than a half million people and displaced millions.
While government forces have made heavy use of artillery and rockets in the assault, rebels said Russian warplanes have not as yet taken part.
Still, Russia's ambassador to Lebanon was quoted as saying on June 22 that Russia was helping Damascus to retake territory in the south.
"The Syrian Army now, with support from Russian forces, is recovering its land in the south and restoring the authority of the Syrian state," Aleksandr Zasypkin told the pro-Hizballah newspaper Al-Akhbar.
Assad has this year recaptured the last remaining enclaves of insurgent territory near the capital, Damascus, and the city of Homs, including the densely populated eastern Ghouta region.
But there are still large areas outside his control. Apart from the southwest, rebels also hold a swathe of northwest Syria. Insurgent groups backed by Turkey hold parts of the northern border area.
And the quarter of Syria east of the Euphrates is controlled by an alliance of Kurdish and Arab militias supported by the United States. The United States also has a base at Tanf, near Syria's borders with Iraq and Jordan, which controls the Damascus-Baghdad highway.