A flurry of leaks from the Pentagon and US state department accusing Syria of preparing to use chemical weapons is being used by the Obama administration to underpin threats of military action against the regime of Bashar al-Assad.

The US claims have been met with incredulity by Syria's principal ally, Russia, which has suggested they are being used as a pretext to increase pressure on Assad and to prepare for the use of force.

In recent days, NBC News has reported anonymous US officials as saying that Syria has loaded chemical weapons into bombs and that Assad is prepared to use them against his own people.

The New York Times quoted anonymous officials as saying the Syrian military has moved chemical weapons in preparation for their use. One official said: "The activity we are seeing suggests some potential chemical weapon preparation."

Wired magazine's Danger Room blog reported that the Assad regime has "begun combining the two chemical precursors needed to weaponize sarin gas".

CNN on Friday quoted a Pentagon official as saying the US had updated its military options for a potential strike against Syria. The official told CNN the US "has all the firepower it needs in the region" to attack Syria if Barack Obama orders it.

The leaks come as the US defence secretary, Leon Panetta, and the secretary of state, Hillary Clinton, step up warnings to Syria over alleged chemical weapons.

"I think there is no question that we remain very concerned, very concerned that as the opposition advances, in particular on Damascus, that the regime might very well consider the use of chemical weapons. The intelligence that we have causes serious concerns that this is being considered," said Panetta. "The president of the United States has made very clear there will be consequences, there will be consequences if the Assad regime makes a terrible mistake by using these chemical weapons on their own people."

Clinton said Assad would cross "a red line" if he used chemical weapons. She said Washington was concerned "an increasingly desperate Assad regime might turn to chemical weapons, or might lose control of them to one of the many groups that are now operating within Syria".

It is not clear if the Pentagon and state department leaks are co-ordinated, but they have had the effect of sharply increasing the pressure on Syria and its allies, particularly Russia, which has resisted outside intervention in the crisis.

The Russian foreign minister, Sergei Lavrov, this week accused Washington of inventing a non-existent chemical weapons threat as a pretext for military action – a charge that has echoes of US claims about Iraq before the 2003 invasion.

"As soon as we get these rumours [about chemical weapons] we engage in constructive démarche; when we get confirmation that nothing of that type is happening we share this information with our American colleagues," Lavrov said.

The alleged chemical weapons threat has been used by Nato's secretary general, Anders Fogh Rasmussen, to justify deploying Patriot missiles in neighbouring Turkey.

NBC quoted US officials as saying the Syrian government had ordered its chemical weapons corps to "be prepared" which Washington interpreted as a directive to begin bringing together the components needed to turn Syria's chemical stockpiles into weapons.

"The military has loaded the precursor chemicals for sarin, a deadly nerve gas, into aerial bombs that could be dropped on to the Syrian people from dozens of fighter-bombers, the officials said," according to NBC. The military is "awaiting final orders" from Assad to use chemical weapons, US officials told NBC.

CNN quoted a US official as saying they had received fresh intelligence last week "when satellite imagery showed the movement of trucks and vehicles at sites where chemicals and weapons were stored". "We assume the aircraft are in close proximity to the munitions," the official said.

Other leaks report that the US is training Syrian rebels to secure chemical weapons they may capture (http://alpha.syriadeeply.org/2012/12/to-secure-chemical-weapons-us-trains-rebels-brigades/£.UMIFkrvi_C9) and that the Pentagon has tens of thousands of troops in the region ready to be deployed in Syria if needed.