The UN endorsement, which came on Wednesday after opposition groups demanded international intervention, was likely to deflate their calls for the elections to be cancelled.
Final results are expected to be announced next week.
Preliminary results, which gave a big lead to the ruling Shia religious bloc, also indicated that Ahmad Chalabi, an Iraqi deputy prime minister and former Washington insider, will not be re-elected to the new 275-member parliament, his office said.
The United Nations official, Craig Jenness, said at a news conference organised by the Independent Electoral Commission of Iraq (IECI) that his international election assistance team found the elections to be fair.
"The United Nations is of the view that these elections were transparent and credible," said Jenness, a Canadian electoral expert.
"The United Nations is of the view that these elections were transparent and credible"
Jenness said the number of complaints was less than one for every 7000 voters. About 70% of Iraq's 15 million voters went to the polls.
His remarks represented support for Iraqi election commission officials, who refused opposition demands to step down. They, too, said that the elections were free and fair and that they would deal with the few instances of fraud and rigging of ballot boxes.
Safwat Rashid, an IECI official said: "No wide, premeditated and systematic fraud was noticed."
The Bush administration and many Iraqi officials hope the elections will lead to a broad-based government that will include minority Sunni Arabs as well as secular Shia such as Iyad Allawi, the former interim prime minister.
Jenness said: "In our view, all communities of Iraq have won in these elections, all will have a strong voice in parliament. We hope the elections will be the start of a new process of strength and unity in Iraq."
Also on Wednesday, more than 4000 people rallied in Samarra, a predominantly Sunni Arab town 100km north of Baghdad.
Demonstrators carried banners reading, "We refuse the election forgery".
"The UN stand provokes our astonishment because they have not responded to our complaints, which we have submitted ... [the UN should] check our complaints and then express its views"
Prominent Sunni candidate Saleh al-Mutlaq, who has joined forces with Allawi's secular group to protest against what they have described as rampant fraud, said he was angered by Jenness' remarks.
He again demanded an independent review of about 1500 complaints, including 50 or so deemed serious enough to affect the results in some areas.
"The UN stand provokes our astonishment because they have not responded to our complaints, which we have submitted," al-Mutlaq said. "This statement provokes anger and frustration."
He said the UN should "check our complaints and then express its views".
Instances of fraud
Iraqi officials said they had found some instances of fraud that were enough to cancel the results in some places but not to hold another vote in any district.
"After studying all the complaints, and after the manual and electronic audit of samples of ballot boxes in the provinces, the electoral commission will announce within the next few days some decisions about cancelling the results in stations where fraud was found," said Abdul Hussein Hendawi, an elections official.
Ibrahim al-Jaafari is a likely
alliance candidate for premier
He said fraud was discovered in the provinces of Baghdad, Irbil, Ninevah, Kirkuk, al-Anbar and Diyala.
Preliminary results from the vote have given the governing Shia religious bloc, the United Iraqi Alliance, a big lead, but one that still would require forming a coalition with other groups.
The bloc held further talks with Kurdish leaders on Wednesday and said preparations were being made to choose a candidate for prime minister, whom they have said must come from the United Iraqi Alliance.
Alliance officials have indicated the likely candidates for prime minister are Ibrahim al-Jaafari, the current prime minister who leads the Islamic Dawa party, and Adel Abdul-Mahdi, who belongs to the other main Shia party, the Supreme Council of the Islamic Revolution in Iraq.