Secretary of State Hillary Clinton sternly warned Syria Monday that using chemical weapons against rebel forces would be a "red line" and trigger retaliatory action from the United States.

Along with a pledge from President Obama, Clinton's words were the second public warning from the U.S. government in response to reports that Syria's President Bashar al-Assad may be moving a chemical weapons stockpile.

"We have made our views very clear. This is a red line for the United States. I'm not going to telegraph in any specifics what we would do in the event of credible evidence that the Assad regime has resorted to using chemical weapons against his own people, but suffice to say we are certainly planning to take action if that eventuality were to occur," Clinton said at a press conference in Prague.

A U.S. official confirmed to ABC News that late last week U.S. intelligence determined that Syria, in addition to moving some of its chemical stockpiles, had taken preparatory steps with regards to some of the stockpile.

The concern is over the movement of weapons that could be used to contain sarin gas - a fast but short-acting nerve agent, according to another U.S. official. He said attention is focused on the number of items the Syrians moved and what those items contained. Congress was notified over the weekend about this development.

Clinton said the U.S. was sending "a very strong warning to the Assad Regime."

"Their behavior is reprehensible, their actions against their own people have been tragic, but there is no doubt there is a line between even the horrors that they have already inflicted on the Syrian people and moving to what would be an internationally condemned step of utilizing their chemical weapons."

People exposed to large amounts of sarin may black out, start seizing or even stop breathing, according to the Centers for Disease Control.

In Washington on Monday President Obama called the possible use of chemical weapons "totally unacceptable."

"I want to make it absolutely clear to Assad and those under his command, the world is watching," Obama said at the Nunn-Lugar Cooperative Threat Reduction symposium.

State Department Spokesman Mark Toner echoed Clinton's red line remarks.

"What we've been very clear about, though, is that, as we said, any user proliferation would be crossing a red line, and we would take necessary steps or actions," Toner told reporters. "I'm just not going to get into any specifics about their chemical weapons program - only that we are concerned about any move that might signal that they are somehow ready to use those chemical weapons on their own people."

Sarah Parnass contributed to this report