Syria's government has recaptured half of the shrinking rebel enclave with air strikes that killed dozens Wednesday in defiance of United Nations calls for a halt to violence.

"The Russians made a deliberate choice not to restrain the Assad regime. Thus, the carnage in East(ern) Ghouta continues. Russia is morally complicit and responsible for Assad's atrocities," Pentagon spokeswoman Dana White told reporters.

"We urge Russia to compel the Assad regime to stop killing innocent Syrians and allow much needed aid to reach the people of East(ern) Ghouta and other remote areas. We support our diplomats in their efforts to resolve the conflict with the United Nations-backed Geneva process."

The 15 UN Security Council members -- including the United States -- unanimously adopted a resolution last month demanding a monthlong ceasefire. But it has not been respected.

The Russia-backed Syrian army and allied militia began a fierce bombing campaign on February 18 on Eastern Ghouta -- the last opposition bastion near Damascus -- then launched a ground offensive a week later.

Russia, which was among those voting in favor of the text, indicated Thursday it would continue to back Damascus in its military offensive.

"We will continue fighting terrorists, we will finish them off, we will help finish them off in Eastern Ghouta, where the Syrian army is now conducting operations with our support," said Russian Foreign Minister Sergei Lavrov.

The assault on Ghouta has left nearly 1,250 civilians dead, around a fifth of them children.

"Russia is either incompetent, committing illegal acts or both. Russia is propping up the Assad regime, not targeting Al-Qaeda affiliated terrorists in Syria. Russia could stop the civilian casualties in Syria," said White.