The United States has started work on the construction of a test site for its controversial land-based missile defence system.

We will not allow the world's most dangerous regimes to threaten us with the world's most dangerous weapons

Paul Wolfowitz, US deputy defence Secretary

A set of silos is being built in Alaska to house interceptors capable of destroying missiles fired by what the US calls rogue states.

The work started two days after the US unilaterally withdrew from the 1972 Anti-Ballistic Missile (ABM) treaty which prohibited such initiatives.

"We are now free to develop, test and deploy effective defences against missile attack from states like North Korea and Iran," US Deputy Defence Secretary Paul Wolfowitz wrote in a letter published in The Wall Street Journal.

"As the president said in his State of the Union address, we will not allow the world's most dangerous regimes to threaten us with the world's most dangerous weapons." The silos, initially classified as test facilities, are expected to be fully operational by 2007.

Test bed

US military officials gathered at Fort Greely in central Alaska on Saturday to break the ground at a site for six land-based interceptor missiles.

The intention is that the six silos will be built and operational in just over two years, though they will still be classified as test facilities.

So far existing tests have had only mixed results

Mr Wolfowitz wrote in his letter that the purpose of the Alaska site was to "provide the facilities necessary to test and evaluate a wide range Ground-based Midcourse Defence (GMD) support programmes and systems".

BBC correspondent Justin Webb in Washington says only after 2007 would the silos - or modified systems based on their designs - be expected to work under realistic conditions.

'Decades away'

Some American experts express deep scepticism about whether or not this hugely expensive project will ever provide a genuine defence against a missile attack.

Experts say US is "decades away" from the proposed systems

They say progress so far has been very slow and existing tests have had only mixed results, despite a successful firing of an interceptor rocket in the Pacific Ocean on Thursday.

Leading scientists believe that the US is decades away from the sort of multi-layered defensive system proposed by President Bush.

But our correspondent says that scepticism is brushed aside by the Bush administration.

He says the ceremony is further evidence that the president is serious about missile defence.