Assad’s military has loaded the precursor chemicals for sarin, a deadly nerve gas, into bombs that could be dropped onto the Syrian people from dozens of fighter-bombers, the officials said.
As recently as Tuesday, U.S. officials had said there no evidence yet that mixing the precursor chemicals had begun. But Wednesday, they said their worst fears had been confirmed: The nerve agents were locked and loaded inside the bombs.
Sarin is extraordinarily lethal. Saddam Hussein's forces killed 5,000 Iraqi Kurds with a single sarin attack on Halabja in 1988.
The officials speaking to NBC stressed that the sarin bombs hadn't yet been loaded onto planes and that Assad hadn't issued a final order to use them. But if he does, one said, "there's little the outside world can do to stop it."
Secretary of State Hillary Rodham Clinton reiterated U.S. warnings to Assad Wednesday not to use chemical weapons, saying he would be crossing "a red line" if he did so.
Speaking at NATO headquarters in Brussels, Clinton said "an increasingly desperate Assad regime," near its downfall, might turn to chemical weapons or that the banned weapons could fall into other hands.
In contrast, Israeli Vice Prime Minister Moshe Yaalon said Wednesday that Assad has responded to warnings about the security of chemical arms by keeping them out of extremists’ hands, Reuters reported.
Yaalon joined Prime Minister Benjamin Netanyahu in making clear that Israel is as concerned about chemical weapons falling into the hands of the rebels as it is about them being used by state forces in the Syrian civil war.
"Together with the international community, we are closely monitoring developments in Syria regarding its stores of chemical weapons," Netanyahu said Tuesday. "Such weapons must not be used and must not reach terrorist elements."
In an interview with the Israeli news website Walla that was posted on his Facebook page, Yaalon said: "There is speculation that the chemical arsenal will fall into the hostile and irresponsible hands of the likes of al Qaeda or other terrorist groups.
"In the past, clear messages were relayed to Assad on a number of opportunities, and in response Assad in fact gathered up the weaponry and separated the materials," Yaalon said.
Aides told NBC News that Clinton was expected next week to officially recognize the main opposition movement, the National Coalition of Syrian Revolutionary and Opposition Forces, with which she is scheduled to meet in Morocco.
In fighting in Syria, rebels said Wednesday they had surrounded the Aqraba air base, about 4 km (2.5 miles) outside Damascus.
"We still do not control the air base but the fighters are choking it off. We hope within the coming hours we can take it," Abu Nidal, a spokesman for a rebel force called the Habib al-Mustafa brigade, told Reuters.
He said rebels had captured a unit of air defense soldiers, killing and imprisoning dozens while others escaped.
Syria's state news agency said the army was still firmly in control of the base, but did not comment on rebel claims that they were surrounding the area.