"The Syrian army is fully ready, its finger on the trigger to face any challenge or scenario that they want to carry out," Prime Minister Wael al-Halqi said.
And a security official, asking not to be named, told AFP that Syria was now "expecting an attack at any moment."
The White House said Obama would speak to the American people at 1715 GMT, but would not announce imminent military action.
Manoeuvering over the past two days has appeared to signal that Obama has called for "limited" military strikes that could take place shortly.
But developments today could indicate an prolongation of the timetable for diplomatic reasons.
Arab League foreign minister, meanwhile, brought forward to Sunday a Cairo meeting that had been scheduled for Tuesday. Earlier this week, it placed the "entire responsibility" for the alleged attacks on Damascus, and any further condemnation of Assad's actions could give Washington extra diplomatic cover for military action.
Furthermore, UN spokesman Martin Nesirky said inspectors who spent four days investigating an alleged chemical attack last week would not issue a report on their findings until laboratory tests are conducted.
With the departure of the inspectors today opening a window for a possible US-led punitive strike, the head of Iran's elite Revolutionary Guards warned that this would trigger reactions beyond the borders of Tehran's key regional ally.
"The fact that the Americans believe that military intervention will be limited to within Syrian borders is an illusion," said commander Mohammad Ali Jafari.
"It will provoke reactions beyond that country," and "reinforce extremism," he said, quoted by the ISNA news agency.
The inspectors, who flew to the Netherlands today, are due to report straight back to United Nations chief Ban Ki-moon and detail their conclusions on whether a poison gas attack actually did take place in Damascus suburbs on August 21.
Obama's administration says it already has firm evidence that the regime launched a chemical onslaught that killed 1,429 people, including at least 426 children.