President Bush has authorised the US Marine Corps to begin the involuntary call-up of thousands of inactive Marines for service in Iraq and Afghanistan to make up a shortfall in the number volunteering for duty.

Under the order, up to 2,500 Marines can brought back to active duty at any one time, but there is no limit on how many may be forced back into service in the coming years or how long the call-up may last.

Marine officials said they had the authority to call Marines back against their will as long as they were needed to serve the "global war on terror" -- a conflict whose parameters remain undefined. "The authority is until GWOT [global war on terror] is over with," Colonel Guy Stratton, head of the Marine Corps' manpower mobilisation plans, said. "Until we're told to do otherwise, we'll use it."

The order is the first time that the Marine Corps have been forced to resort to an involuntary summons since the ground invasion of Iraq in 2003. The Army, far worse hit by recruiting shortfalls, has ordered around 14,000 inactive servicemen and women back to duty since the war in Iraq began.

The move has thrown the spotlight back on to the military's ability to prosecute an ill-defined and increasingly unpopular long-term military campaign with an all-volunteer force. The Army has already been criticised for bringing in a form of back-door draft by using measures to stop soldiers from leaving such as temporary "stop loss" orders that suspend or postpone the leaving dates of troops returning from combat.

About 35,000 Marines could be affected by the order. Marines usually serve four years on active duty and another four in reserve. While on reserve, Marines may volunteer to return to active duty to fill needed roles.

However, the number of Marines volunteering outside their active-duty service requirement has been steadily declining for two years, leaving a shortfall of 1,200.

The Marine Corps' move comes almost five years after the September 11 attacks that led the US to declare "a war against terrorism of global reach", and more than three years after the Iraq war began.

Almost half of the 178,000 US Marines have had a tour of duty in Iraq or Afghanistan and many Marines have completed three tours in Iraq.

Marines have carried a heavy load in the fighting there, serving in the most dangerous parts of the country such as Anbar. More than 6,000 Marines have been wounded and 650 killed in Iraq alone.

But the elite corps has also seen its proud image tarnished by incidents such as the alleged massacre of civilians at Haditha and accusations of a cover-up by senior officers.

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