The United States will not tolerate any use of chemical weapons in Syria and will act quickly if that threat appears imminent, Secretary of State Hillary Rodham Clinton said Monday, as heavy fighting continued around the Syrian capital.

Syria’s Foreign Ministry said the government planned no such escalation of the 20-month civil war. …

“This is a red line for the United States,” Clinton said. “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 their own people. But suffice to say we are certainly planning to take action.”