The Pentagon harshly criticized Russia on Monday for its bombing in Syria, and said its indiscriminate bombing campaign makes any future deal to cooperate in the battle against the Islamic State far more difficult.

"We are not there, and the regime and Russia's recent actions only make it harder to consider any potential coordination," said Pentagon Press Secretary Peter Cook.

Russia and the U.S. have been discussing cooperating on a plan designed to end the bombing of Aleppo, open humanitarian corridors and move toward a political settlement to end the civil war.

But the Pentagon said little progress has been made since Secretary of State John Kerry met with his Russian counterpart Sergey Lavrov last month in Moscow.

Instead, the U.S. said Russia and the Syrian regime of Bashar Assad have intensified the bombing of Aleppo, with a total disregard for the suffering of the civilian population.

Cook accused Russia of bombing densely populated urban areas, interrupting water and electrical services and maiming civilians, and said those actions are only adding fuel to Syria's civil war and doing nothing to degrade extremist groups.

"It is long past time for Russia to protect civilians," Cook said at a Pentagon briefing. "We will not commit indefinitely to discussions that do not lead to results."

The Syrian Network for Human Rights charged this week that Russia's reckless bombing campaign has killed more civilians than the Islamic State.

The United Nations-sanctioned human rights group claims Russian bombs have killed at least 2,704 civilians, while the Islamic State has murdered at least 2,686.

Cook cited Russia-intensified bombing as one reason no deal is in the offing. "Contrary to recent claims, we have no finalized plans with Russia on potential coordinated efforts. Serious issues must first be resolved," he said.