Trying to stop the spread to countries like Iran of medium- and long-range missiles, which could be armed with nuclear or biological warheads, is a top Clinton administration priority.

At the same time, the admininstration's critics in the US Congress say it is not doing enough either to stem the proliferation of such weapons, or to develop possible defences against them.

In terms of that debate, this test is potentially significant. It is also likely to revive and reinforce concerns about Iran's regional military and strategic ambitions.

Iran has long had a missile arsenal. It exchanged barrages of Scud missiles with Iraq in their eight-year conflict in the 1980s. The new missile, dubbed Shahab Three by Tehran, has much greater range - up to 1,300 kilometres.

That could allow it to threaten Israel and the whole Gulf including Saudi Arabia. It is actually a North-Korean-designed weapon, or a very close Iranian copy.

And some analysts argue that there is an increasingly well-developed network of countries clandestinely co-operating to produce missiles, despite US-led efforts to try to stop them.

In April, ahead of its nuclear tests, Pakistan test-fired its Ghauri missile, which is thought to be very similar to the Iranian and North Korean weapons.

Washington also suspects Russia and China, as well as North Korea, of helping Iran's missile programmes, and this has complicated Washington's relations with Moscow and Beijing too.

The United States says the spread of missiles in a region like the Middle East is so destabilising because it allows so-called "rogue states" to threaten their neighbours with weapons of mass destruction, and there is very little defence against them once they're fired.

Israel, which clearly takes the missile threat very seriously, is trying, in collaboration with the United States and with US financial assistance, to develop a system to defend against them.

But Iran is also thought to be developing a much longer-range missile than the Shahab Three. And analysts say the next stage in the threat is that missiles will not just threaten stability in a particular region like the Middle East, but allow regional states to "break out" and threaten the United States and its allies directly.