Chief United Nations weapons inspector Hans Blix has said Baghdad's promise to destroy missiles is "very significant".
Inspectors say the missiles must start being destroyed on Saturday
Mr Blix ordered the destruction of the al-Samoud II weapons system because it breaches limits on Iraqi missiles imposed by the UN.
Iraq agreed to start destroying the missiles by Saturday - a deadline laid down by Mr Blix which was being seen as a key test of Baghdad's willingness to disarm.
Both the US and the UK dismissed the Iraqi offer as another example of Baghdad playing with the weapons inspectors.
But Mr Blix said: "This is a very significant piece of real disarmament."
Tactical surface-to-surface ballistic missile powered by liquid fuel
Tested at range of 183 km - in excess of UN 150 km limit
Diameter also in excess of prescribed limit
May be able to deliver biological or chemical warhead
He cautioned that he still needed clarification on the details of the destruction of the al-Samoud system, which has been test-fired beyond the 150-kilometre (93-mile) limit for Iraqi missiles imposed after the 1991 Gulf War.
But Mr Blix's reaction on Friday was markedly different to the language of a draft of his forthcoming report to the UN obtained by the BBC when he said inspections had produced "very limited results".
He formally submitted that report - said to be very similar to the draft - to the UN on Friday evening New York time.
Mr Blix said that his report had been largely overtaken by the Iraqi decision in principle to destroy the al-Samoud missiles.
The US administration - which with Britain is warning that force may be the only way to ensure Iraq has no weapons of mass destruction - disagreed with Mr Blix's latest assessment.
A spokesman for President George W Bush said the destruction pledge was mere "deception" and repeated that the US wants "complete and total disarmament".
UK Prime Minister Tony Blair was equally sceptical. He declared he knew Baghdad would comply with the UN order before Mr Blix reported to the Security Council on Saturday on progress in Iraq.
"The moment I heard earlier in the week that Saddam Hussein was saying he would not destroy the missiles was the moment I knew that later in the week he would announce, just before Dr Blix reported, that he would indeed destroy these missiles," he said.
But French Foreign Minister Dominique de Villepin said it was an important step in the disarmament of Iraq and confirmed that the inspections produced results.
The Iraqi offer was made in a letter to Mr Blix by Saddam Hussein's scientific adviser who said he agreed "in principle" to the destruction of the missiles.
General Amir al-Saadi requested a UN technical team to discuss the "framework and timetable" for the task; deputy executive chairman of the UN inspectors, Demetrius Perricos, is already in Baghdad to oversee the process.
"There will be technical discussions between Unmovic and Iraq on Saturday morning following which the destruction process could start," Hiro Ueki, spokesman for the inspectors, told
Reuters news agency.
The BBC's Paul Wood in Baghdad says informed sources in the Iraqi capital said the letter also contained some angry words.
Baghdad complained that the inspectors had ignored the scientific and technical facts about the missile and repeated a demand for more tests on the missiles to determine whether they violate the limit.
Despite this protest, our correspondent says the Iraqis concluded that open defiance of Mr Blix was not an option while the UN Security Council is considering the resolution which would trigger a war.
In other developments:
The US orders a sixth aircraft carrier, the Nimitz, to the Gulf, where more than 200,000 troops are now posted; B-2 stealth bombers are also being deployed for the first time
- Russia's Foreign Minister Igor Ivanov said Moscow would use its UN Security Council veto if necessary to preserve "international stability", adding Russia was opposed to a "power solution" in Iraq.