Allied intelligence have detected Syrian movement of chemical weapons components in recent days, a senior US defence official has said, as the Obama administration warned the Assad regime against using them.
The senior official said intelligence sources have detected activity around more than one of Syria's chemical weapons sites in the last week.
Secretary of State Hillary Clinton, in Prague for meetings with Czech officials, reiterated president Barack Obama's declaration that Syrian action on chemical weapons is a "red line" for the United States that would prompt action.
"We have made our views very clear: this is a red line for the United States," she told reporters.
"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 it to say, we are certainly planning to take action if that eventuality were to occur."
Syria's Ministry of Foreign Affairs said the regime "would not use chemical weapons - if there are any - against its own people under any circumstances".
Syria has been careful never to confirm that it has any chemical weapons. The use of chemical weapons would be a major escalation in Assad's crackdown on his foes and would draw international condemnation. In addition to causing mass deaths and horrific injuries to survivors, the regime's willingness to use them would alarm much of the region, particularly neighbouring states including Israel.
Syria is believed to have several hundred ballistic surface-to-surface missiles capable of carrying chemical warheads.
Earlier on Monday Syria bombed a security building that had been taken over by rebels on the Turkish border, wounding at least 11 people and sending dozens of civilians fleeing across the frontier.
Lebanese soldiers exchanged fire with Syrian rebels across their border, fuelling concerns that the Arab Spring's longest and deadliest revolt could spark a regional war. The violence came as Russian president Vladimir Putin headed to Turkey for talks likely to be overshadowed by the two countries' differences over Syria.