By Jonathan Marcus BBC defence correspondent

US Vice-President Dick Cheney has provided the most forceful rationale yet for an attack against Iraq.

A war with Iraq is a risky option which America's friends and allies will approach with some trepidation

He argued that Washington had to act soon and could not wait for Iraq to acquire nuclear weapons.

His remarks come at a time when there is a growing debate in the US on the wisdom of any attack.

White House legal advisors are reported to have told the administration that President George W Bush could go to war without necessarily seeking a mandate from Congress.

But building a wider consensus may be vital if a military operation is to be launched.

Making a case

Just a few days ago it looked as though the Bush administration was backing away from its war plans against Iraq.

Dick Cheney issued his most hawkish speech so far

A stream of critical voices from the foreign policy establishment and Republican supporters on Capitol Hill - among them many advisers of President Bush's own father - appeared to have caught the Bush team off balance.

But rather than backing away, it looks as though the president's supporters have simply been girding up for the struggle ahead.

Many critics argue that the case has not yet been made for an attack.

Mr Cheney's comments are the administration's first real effort to begin to make such a case.

Building consensus

However, presenting arguments is one thing, building up a real consensus behind a war plan is quite another.

Scowcroft said a strike "could unleash an Armageddon in the Middle East"

President Bush's legal experts may be right in a narrow sense when they say that he does not need to get a congressional vote to back a war against Iraq.

But whether this makes good political or diplomatic sense is quite another matter.

A war with Iraq is a risky option which America's friends and allies will approach with some trepidation.

If the Bush adminsitration is to build wider support for its cause it needs to first build consensus at home.

A real congressional debate may be essential in building that consensus.

Experts are already pointing to the build-up to the first Gulf War in 1991 when President Bush senior - while insisting such a resolution was not necessary - did indeed seek and obtain congressional support for the war to oust Iraqi troops from Kuwait.

And - in a further sign that the administration is serious about its plans to remove Saddam Hussein - some 14,000 US military reservists are being notified that their call-up period could be extended for up to two years.

This is the first such extension since the Vietnam War era.