Those responsible would be held accountable, the statement said.
Iran had previously rejected suggestions that one of its missiles brought down the plane near the capital, Tehran, on Wednesday.
But pressure mounted after the US and Canada, citing intelligence, said they believed Iran had shot down the plane with a missile, possibly accidently.
The downing of Ukraine International Airlines flight PS752 with the loss of 176 lives came just hours after Iran carried out missile strikes on two airbases housing US forces in Iraq.
The Ukrainian flight, which was headed to the Canadian city of Toronto via the Ukrainian capital, Kyiv, came down near Imam Khomeini Airport shortly after take off.
In a tweet on Saturday, Iranian foreign minister Mohammad Javad Zarif suggested “US adventurism” was partly to blame for the downing of the Ukrainian jet.
As the evidence pointing to a missile strike built, Iran promised a full investigation. But on Thursday, TV images from the crash site showed a mechanical digger helping to clear debris away, raising concerns that important evidence could have been removed.