U.S. and allied intelligence have detected Syrian movement of chemical weapons components in recent days, a senior U.S. defence official said Monday, as the Obama administration again warned the Assad regime against using chemical weapons on Syrian rebels.

A senior defence official said intelligence officials have detected activity around more than one of Syria's chemical weapons sites in the last week. The defence official spoke on condition of anonymity because he was not authorized to speak publicly about intelligence matters.

U.S. President Barack Obama warned Syria on Monday that the use of chemical weapons would be "totally unacceptable" and that the country's leaders would be held accountable.

Obama said that if Syrian President Bashar Assad made the "tragic mistake" of deploying chemical weapons, there would be consequences. Obama stopped short of detailing those consequences.    

Obama's comments came as U.S. officials said intelligence had detected Syrian movement of chemical weapons components in recent days.    

U.S. Secretary of State Hillary Rodham Clinton, in Prague for meetings with Czech officials, reiterated President Barack Obama's declaration that Syrian action on chemical weapons was a "red line" for the United States that would prompt action.

'We once again issue a very strong warning to the Assad regime that their behaviour is reprehensible, their actions against their own people have been tragic.'—U.S. Secretary of State Hillary Clinton

"We have made our views very clear: This is a red line for the United States," Clinton 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."

She didn't address the issue of the fresh activity at Syrian chemical weapons depots, but insisted that Washington would address any threat that arises.

White House press secretary Jay Carney said U.S. officials were closely monitoring Syria's proliferation of sensitive materials and facilities, as opposition to the Syrian government grows.    

Obama spoke later at a gathering on securing nuclear weapons materials.  

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Air travel unsafe

A commercial jet on its way to the Syrian capital turned back because of violence near Damascus airport, officials at Cairo's international airport said on Monday.

The officials said the Egypt Air flight from Cairo was rerouted about 30 minutes after takeoff when Egyptian officials received word from their counterparts in Damascus that the area near the airport was not safe.

The Syrian government did not immediately comment.

Syria is believed to have several hundred ballistic surface-to-surface missiles capable of carrying chemical warheads.

Its arsenal is a particular threat to the American allies, Turkey and Israel, and Obama singled out the threat posed by the unconventional weapons earlier this year as a potential cause for deeper U.S. involvement in Syria's civil war.

Up to now, the United States has opposed military intervention or providing arms support to Syria's rebels for fear of further militarizing a conflict that activists say has killed more than 40,000 people since March 2011.

Clinton said that while the actions of President Bashar Assad's government have been deplorable, chemical weapons would bring them to a new level.

"We once again issue a very strong warning to the Assad regime that their behaviour is reprehensible, their actions against their own people have been tragic," she said. "But there is no doubt that there's a line between even the horrors that they've already inflicted on the Syrian people and moving to what would be an internationally condemned step of utilizing their chemical weapons."

Lebanese soldiers, Syrian rebels exchange fire

In a separate development, Lebanon's state news agency reports that Lebanese soldiers have exchanged fire across the border with rebels in neighbouring Syria.

The state-run National News Agency said late Sunday that Lebanese soldiers stationed near the village of Qaa in the Bekaa Valley returned fire into Syria after "armed men" shot at them from across the frontier.

The agency quoted a statement from the Lebanese army that said the exchange of fire took place Saturday and that there were no casualties.

Since it began in March 2011, Syria's civil war on several occasions has spilled into neighbouring countries, including Turkey, Israel and Lebanon, raising fears that the Arab Spring's longest and deadliest revolt against the authoritarian regime in Damascus could spark a regional war.