The scale and purpose of the operation against Syria has not changed in recent weeks, although US forces would adjust as needed, a defense official told AFP.

"We will continue to review our targeting and targeting options as the Syrian government adapts over time," said the official, who spoke on condition of anonymity.

"We are working to the same objective that President (Barack) Obama has outlined," he insisted, responding to a report in the Los Angeles Times Sunday

The strikes against the regime of Syrian President Bashar al-Assad, in retaliation for what the US says is the regime's use of chemical weapons in a Damascus suburb, could last longer than a day, officials have said.

The Los Angeles Times had reported Sunday the Pentagon was readying more intense and longer attacks on Syria than originally planned, set to last three days.

War planners now aim to unleash a heavy barrage of missile strikes to be followed swiftly by additional attacks on targets that may have been missed or remain standing after the initial launch, the Times cited officials as saying.

Two US officers told the newspaper that the White House asked for an expanded target list to include "many more" than the initial list of around 50 targets.

The move is part of an effort to obtain additional firepower to damage Assad's dispersed forces.

The top US military officer, General Martin Dempsey, told lawmakers last week there would be an "initial" set of targets and then a secondary list of targets.

Dempsey suggested American forces would be able to shift strike plans even as the Syrian regime attempts to disperse equipment.

Pentagon planners are now considering using Air Force bombers, as well as five US missile destroyers currently patrolling the eastern Mediterranean Sea, to launch cruise missiles and air-to-surface missiles from far out of range of Syrian air defenses, according to the newspaper report.

The USS Nimitz aircraft carrier strike group with one cruiser and three destroyers positioned in the Read Sea can also fire cruise missiles at Syria.

"There will be several volleys and an assessment after each volley, but all within 72 hours and a clear indication when we are done," an officer familiar with the planning told the Times.

The intensified military planning comes as Obama prepares to personally make his case to the American people and further press reluctant lawmakers on the need for action after Assad allegedly used chemical weapons on his own people last month.

Obama is scheduled to tape interviews Monday with anchors of the three major broadcast networks, as well as with PBS, CNN and Fox News.

The interviews, to air that night, will precede Obama's address to the nation Tuesday ahead of an expected full Senate vote.

The president favors a limited attack with only a reduced number of warplanes to drop bombs over Syria, according to the Times.

Amid doubts a limited US offensive would sufficiently hamper Assad's military capabilities, one officer told the newspaper the planned operation would amount to a "show of force" over several days that would not fundamentally change the situation on the ground.

The planned US strike "will not strategically impact the current situation in the war, which the Syrians have well in hand, though fighting could go on for another two years," another US officer said.