The United States said that Russia's air war in Syria had "dangerously exacerbated" the conflict, accusing Moscow of seeking to bolster President Bashar al-Assad's regime instead of targeting jihadists.
Washington's latest broadside against Russian intervention in Syria came as government troops, emboldened by Moscow's support over the past month, recaptured from the Islamic State group a key road into second city Aleppo.
It also came as Russian air force commander, Viktor Bondarev, said Thursday Moscow had sent anti-aircraft missile systems to Syria to back up its air campaign.
"We sent there not just fighter planes, strike aircraft and helicopters but also anti-aircraft rocket systems," Bondarev told Komsomolskaya Pravda tabloid daily.
He said that Russia made the decision to bring missile systems to Syria because "we took into account every possible threat."
Bondarev said Russia has "more than 50 planes and helicopters" in Syria, "precisely the number we need. At the moment, we do not need more."
Speaking to the House Foreign Affairs Committee, US Assistant Secretary of State for Near Eastern Affairs Anne Patterson said the offensives, backed by Russian air strikes, had displaced at least 120,000 people.
"Russia's military intervention has dangerously exacerbated an already complex environment," she said.
"Moscow has cynically tried to claim that its strikes are focused on terrorists, but so far, 85 to 90 percent of Syrian strikes have hit the moderate Syrian opposition and they have killed civilians in the process," Patterson said.
Civilians have died in Russian strikes on civil defense crews, hospitals, centers for displaced persons and ambulances, she claimed.
"We know that Russia's primary intent is to preserve the regime," she said.
Regime forces launched major offensives in several parts of Syria after Russia began its intervention on September 30, with more than 1,300 air strikes carried out so far.
"Despite our urging, Moscow has yet to stop the Assad regime's horrific practice of barrel bombing the Syrian people," Patterson said.
She said the situation called for a "full court press to end the war and get to a political settlement."
"The Russian deployments cannot be used to stiffen the Assad regime's resistance to a political transition."
- UN urges talks -
In recent weeks, diplomatic efforts have been stepped up to resolve the conflict, which has left more than 250,000 dead and forced millions from their homes since March 2011.
Nineteen key international players met for talks in Vienna Friday, including the United States, Iran and Saudi Arabia, in the broadest push yet to end the conflict.
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The meeting, which did not include any representatives of the Syrian government or its opponents, agreed to ask the United Nations to broker a peace deal between the regime and opposition to clear the way for a new constitution and UN-supervised elections.
Another round of international talks could be held in Vienna within two weeks, according to Patterson.
US Secretary of State John Kerry and his Russian counterpart Sergei Lavrov spoke by phone on Wednesday to discuss the "logistics" of those talks, according to state department spokesman John Kirby.
UN Syria envoy Staffan de Mistura met with Lavrov in Moscow on Wednesday as part of the peace push.
Speaking after their talks, de Mistura said the United Nations was ready to bring together representatives of both sides -- the government and the opposition -- for talks in Geneva.
"We are ready, the UN is ready, to start immediately with this process in Geneva," de Mistura said. "When? As soon as possible."
- Major offensives -
On Wednesday, the Syrian army recaptured the sole route into the government-held side of Aleppo city, effectively breaking a nearly two-week siege.
It gained full control of the route from the regime-controlled west of Aleppo to Khanasser, Ithriya and Al-Salmiyeh "after eliminating a number of Daesh (IS) terrorists," state television said.
Advancing IS forces cut off the route at the end of October.
State TV said the road was now being demined and would reopen to traffic Thursday.
The Syrian Observatory for Human Rights, a Britain-based monitor, said the frontline with IS fighters had been pushed back about 10 kilometers (six miles) from the route.
Aleppo, once Syria's economic hub, has been ravaged by years of fierce fighting.
The city has been divided between government control in the west and rebels in the east since shortly after fighting there began in mid-2012.
Regime forces have launched offensives in Aleppo, Hama, Homs and Latakia provinces since the Russian air campaign began, but registered only modest advances.
They have also continued to pound the rebel-held Eastern Ghouta area outside Damascus.
The Observatory said Wednesday at least 12 people were killed in government shelling on Douma in the rebel region.
The town is the frequent target of regime fire, with at least 70 people killed there in government attacks Friday.
Elsewhere, the Observatory said the civilian death toll had risen to 23 in apparent Russian strikes Monday on the IS-held town of Al-Qaryatain.