WASHINGTON (AFP) – Russia is ‘isolated and alone” in accusing Syrian rebels of staging a chemical weapons attack that the United States believes was carried out by the Damascus regime, the Pentagon said Thursday.

All evidence from the August 21 attack indicates that President Bashar al-Assad’s forces unleashed deadly sarin gas, and dozens of other countries share Washington’s view, press secretary George Little told reporters.

“Russia is isolated and alone in blaming the opposition. We’ve seen no credible reporting that the opposition has used chemical weapons in Syria,” Little said.

“We’ve been joined by more than 30 other countries in declaring that the Assad regime is responsible for the use of chemical weapons on August 21,” he said.

The U.S. Defense Department spokesman dismissed Moscow’s allegation that the rebels had fired the chemical agent.

“It’s common sense that the opposition does not have the capabilities to carry out such a large-scale, coordinated rocket and artillery attack from a regime held-neighborhood targeting a rebel-held neighborhood,” Little said.

He also suggested Russia’s President Vladimir Putin’s credibility was on the line after his government proposed placing Assad’s chemical arsenal under international control.

“President Putin has invested his credibility in transfering Assad’s chemical weapons to international control and ultimately destroying them. The world will know whether Russia can follow through with that commitment,” Little added.

The comments came as top American and Russian diplomats were due to meet in Geneva to discuss Moscow’s proposal to destroy Assad’s banned poison gas stockpile.

President Barack Obama has agreed to postpone a possible U.S.-led military strike on Syria to allow time for talks on the proposal.

Russia, a staunch ally of the Assad regime, has insisted that opposition forces were behind the chemical weapons assault in a Damascus suburb, which is believed to have killed hundreds.

A team of UN inspectors are expected to issue a report as soon as Monday about the alleged attack, after collecting soil, blood and urine samples in Syria.

The inspectors do not have a mandate to conclude who carried out the suspected assault but their account is expected to include a variety of circumstantial evidence that will indicate the Assad regime is to blame, according to a report in Foreign Policy, citing unnamed officials.

Human rights groups have pointed to photos, video and the remains of rockets in the area as proof that strongly suggests the regime carried out the attack while Western governments have in part cited intelligence reports, including intercepted conversations of regime officers, as further evidence.