As US naval commanders declared this evening that American destroyers are “fully ready” to execute what they termed a “vast spectrum of operations” against Syria, the Pentagon was said to be considering a doubling-up of its strike capacity with weapons-bearing bombers and other assets of the US Air Force.

The move suggests a military operation that is greater in scale than has previously been suggested.

Military planners in Washington believe President Bashar al-Assad is using the delay before the launching of strikes to shunt around his own assets, possibly including his chemical stockpiles, to different points across territory under his control, meaning any attack may have to last longer or come in two waves.

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With prospects still shaky for passage of an authorising resolution by Congress, the White House is anxious to avoid any impression of “mission creep”, saying the goal remains to punish Syria for the alleged use of chemical weapons. “This is not open-ended. This is not boots on the ground. This is not Afghanistan. This is not Iraq. This is not even Libya,” Tony Blinken, the deputy national security advisor, said.

Adding to the stakes this evening were deployments by Russia to the eastern Mediterranean. Three naval vessels were spotted crossing the Bosporus straits and steaming toward the region. They were identified as the SSV-201 intelligence ship Priazovye and two landing ships Minsk and Novocherkassk.

Any expansion of US assets to include manned aircraft would imply a wider US response to the chemical attacks, with the prospect of greater damage being inflicted. But it would also imply a greater risk of Syrian counter-measures. An immediate question is whether they would overfly Syrian territory running the gauntlet of anti-aircraft arms. Any limits on US engagement may dissolve if US airmen were shot down.

The Pentagon has declined to comment.

“We have provided a range of options to the President, but we will not discuss the details of those options or our operational planning,” Commander Bill Speaks, a Defence Department spokesman, told The Independent.

Martin Dempsey, the Chairman of the Joint Chiefs, this week lamented leaks of US intentions that may assist the regime as it braces for an attack.

Well advertised, however, is the presence of four destroyers already in place close to Syria. Backing them is the Nimitz Aircraft Carrier group in the Red Sea. The four destroyers between them have the ability to launch 40 cruise missiles into Syria. Targets could include Syria’s chemical weapons delivery systems but also other assets including landing fields, helicopters, command posts and perhaps the Defence Ministry.

The readiness of the US destroyers to lead strikes, which currently awaits the debate in Congress on giving Mr Obama authorisation, was confirmed by top US Admiral Jonathan Greenert. Noting they could carry out a “vast spectrum of operations” he implied they could both launch the attack and respond to any retaliation.

But the Pentagon could significantly broaden its operations with US Air Force assets. They could include B-52 bombers, which can also carry and launch cruise missiles from the air as well as low-flying B1s that are based in Qatar and are fitted with long-range and air-to-surface missiles. Nor might the deployment of B-2 stealth bombers, based in Missouri, be ruled out, sources told the Wall Street Journal.

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Heavy armaments could be needed if the Syrian regime uses reinforced bunkers to protect military assets. It is also thought likely by most analysts that a first wave of cruise missile attacks may have to be followed by secondary attacks to make good on targets that are missed or only partially destroyed. “Unfortunately cruise missiles don’t have the penetrating power needed to get into the places where the chemical weapons are,” Rick Francona, a former Air Force intelligence officer and military analyst noted.

It is suspicion that the administration may be underplaying the full extent of what might unfold – the notion of mission creep – that is weighing on the debate in Congress. It is by no means certain that the House of Representatives, in particular, will back Mr Obama’s call for action. If it declines to give the green light in a vote next week the President will then decide whether to abandon the strikes or order them regardless. “That is exactly what is happening,” Mr Rancona said of mission creep in an appearance on CNN.

“What if we execute this strike and then he (Assad) decides to use chemical weapons again?” Susan Collins, a moderate Republican senator from Maine asked. “Do we strike again? Well that is the definition of further engagement. That is the definition of our becoming deeply involved in a war.”

Hearings already completed on Capitol Hill have highlighted the thin line between strikes that are designed only to punish Assad for using chemical weapons and strikes that could change the balance in the civil war. A first authorising resolution passed late on Wednesday by the Senate Foreign Relations Committee speaks directly to “changing the momentum” in the conflict.

When pressed on the issue by Senator John McCain, a committee member who would like a more robust mission, this week General Dempsey appeared to push back. “I have never been told to change the momentum,” Dempsey said. “I have been told to deter and degrade.”