"The most important point is that they can confirm they have not found any stocks or weapons of mass destruction of any kind," Blix said of the report by CIA adviser David Kay.
"I don't think there are any surprises," he told BBC World Service Radio.
Blix said he doubted whether Iraq posed the "clear and present danger" that prompted London and Washington to invade.
"I am of the view that the action could not be reconciled with the demands of the UN charter," he said.
"The intelligence was not so strong in reality that it (the threat) could be said to be manifest," he said.
UK interprets report
British Foreign Secretary Jack Straw, reacting to the report on Friday, insisted that Saddam Hussein's regime "did indeed pose a current and serious threat."
Speaking on BBC radio, Straw said the group's interim report, released on Thursday in Washington, actually backed up the US and British decision to invade Iraq last March.
"The fact that they've not found weapons ... does not mean that they are not there," he said, adding that "a great deal already has been found in terms of programmes" to develop chemical, biological and nuclear weapons.
Jack Straw said the report supported invasion
Questioned later on Sky News television on whether he still felt Saddam's regime was a clear threat to global security, Straw replied, "There's nothing in this report that undermines that."
But the Iraq Survey Group report was seized upon by opponents of Prime Minister Tony Blair, who was US President George W. Bush's staunchest ally in the showdown with Saddam.
Andrew Murray, chairman of the Stop The War Coalition, which organized huge demonstrations in London against the war, said the report confirmed that the reasons given for going to war were false.
"The prime minister now owes the nation an apology," he said.
Bernard Jenkin, the opposition Conservatives' spokesman on defence issues, said the interim report showed "clear evidence of Saddam Hussein's ambitions to acquire weapons of mass destruction" and his failure to comply with UN demands.
The Conservatives had supported Blair on the need for military action.
"Quite honestly I think it would be better now if both London and Washington were to admit there were no weapons"
Former UK foreign minister
But Jenkins added that the interim report did not eliminate the need for "an independent judicial inquiry" into whether Blair's government had exaggerated the threat of Iraqi weapons in the run-up to war.
Robin Cook, who quit Blair's cabinet in protest at the war, said, "Quite honestly I think it would be better now if both London and Washington were to admit there were no weapons, we are not going to find any weapons."
It would be better, the ex-foreign secretary told Channel Four television, to take the millions of dollars being spent on the Iraq Survey Group and redirect the funds to the country's post-war reconstruction.
Blix urged caution about the evidence given to Kay's team.
"Many Iraqi scientists and technicians will feel that they would have a chance to be rewarded for speaking to the Americans and the British. One should be a little cautious about what they may say," he said.
"Just as defectors seem to have come up with many stories that they expected the interrogators wanted to hear, the same thing may have occurred in Iraq now," he added.
"Therefore I think whatever stories they get from one must be corroborated and must be corroborated on the ground."
Blix also took issue with Kay over the danger of leaving Saddam unchecked. "There is one point he makes that if the armed attack had not occurred in the spring, then these things could have proceeded and developed into something bigger.
That would not have happened "because the Security Council had never intended to abandon the long-term monitoring. The Iraqis would not have been left alone to proceed with whatever they had started," Blix said.
Russian President Vladimir Putin says Iraqis trust him
President Vladimir Putin of Russia also took a dig at the United States over Iraq on Friday, telling an international audience that Iraqis trusted Moscow more than they did Washington.
"The country's (Iraq's) people, of course, have far greater trust in their traditional partners than in, shall I say, those who now control the situation," he told a World Economic Forum meeting.
Russia has for years maintained close relations with Baghdad and was an opponent of the US-led war that toppled Saddam Hussein.
The Kremlin leader was far gentler in tone than UN Secretary-General Kofi Annan who, in blunt criticism of Washington, said earlier the world body could not play a proper political role in Iraq under Washington's terms.