The United States is concerned that Syria may be "cooking up recipes" at multiple sites to arm chemical weapons, a U.S. official said on Tuesday.
CNN reported on Monday that Syrian forces battling rebels in fierce fighting had started combining chemicals that could be used to make deadly sarin gas for weapons.
The official would not detail more on intelligence developments, but said the United States is concerned about possible preparations at more than one chemical weapons plant around Syria.
The official would only say it was a "small number" of facilities where activities are taking place.
The Syrian Foreign Ministry denied that the country has any plans to use chemical weapons, state TV has reported. The government likewise has repeatedly stressed it will not use such weapons, if they exist, against its people under any circumstances.
The U.S. military believes there are 50 chemical weapon and production sites spread across the country with additional storage sites and research centers as well.
Syria is believed to have one of the most advanced chemical warfare capabilities in the region, with the ability to develop and produce agents such as mustard gas, sarin and possibly the VX nerve agent, according to information collected by the Nuclear Threat Initiative, a non-profit group that seeks to reduce the risk of use of chemical, biological and nuclear weapons.
American intelligence shows activity at certain facilities and no evidence that anything had been moved out of them, several senior U.S. military officials said.
While they would not describe the activity, the officials said there was no sign that showed Syria was ready to do anything with chemical weapons.
Officials explained that some types of chemical arms require mixing before they can be weaponized.
None of the officials who spoke to CNN would discuss the issue for full attribution due to the sensitivity of the issue.
The intelligence was strong enough for President Barack Obama to issue a public warning to Syrian President Bashar al-Assad on Monday, according to one of the U.S. officials.
"I want to make it absolutely clear to Assad and those under his command -- the world is watching," Obama said at a speech at the National Defense University in Washington.
"The use of chemical weapons is and would be totally unacceptable. And if you make the tragic mistake of using these weapons, there will be consequences and you will be held accountable," he said.
Despite the concern, military officials say there's no imminent plan for U.S. military action. But the military has been planning for various scenarios and has assets in the region should Obama order a strike.
Officials note that regional allies like Jordan also have military capabilities and have been training, and are standing by.
While officials would not discuss U.S. military options, military officials speaking more generally have said that striking the sites would not be a good idea because ensuing explosions would disperse dangerous chemicals.
Senior U.S. military officials also said Tuesday that there was no indication the situation had deteriorated to the point where Assad would resort to chemical weapons, though rebels continue making advances.
A U.S. official not in the military said that the opposition is "maturing" forcing Assad into a tougher position.
"The regime's territorial control and influence appears to be narrowing. The will to fight looks like it's still there, but Assad's forces are struggling to beat back insurgent gains," the official said.