President Hassan Rouhani wrote on @HassanRouhani: "Armed Forces' internal investigation has concluded that regrettably missiles fired due to human error caused the horrific crash of the Ukrainian plane & death of 176 innocent people.
"Investigations continue to identify & prosecute this great tragedy & unforgivable mistake. #PS752."
Iran claimed it was "involuntary human error" and that those responsible would be held accountable.
One user commented: "It's a terrific watershed moment. Really hoping this destructive domino effect would stop ASAP.
"Every player in this current international tension has innocent blood on his hands already be it Trump, Rouhanni, Service Chiefs, every player!"
Another said: "I'm speechless and heartbroken. May all passengers, crew and the Captain rest in peace. #UkranianPlaneCrash, #IranPlaneCrash."
A user posted pictures of some of the passengers who had died in the tragedy and wrote: "They were murdered. Nothing to mention! #IranPlaneCrash."
One post had a meme of an Iranian leader and US President Donald Trump throwing snowballs at each other.
He wrote: "Iran, shame on you if you can't fight with your father (America). Then don't do this to poor ones. #IranPlaneCrash."
A Twitter user tagged Rouhani's tweet and said: "There is no way the Ukrainian plane #PS752 would have been shot down by #Iran, were orders not provided by regime officials, including IRGC. You need source for blame, look no further than yourself! #IranPlaneCrash."
Another tweet went thus: "Some are lauding #Iran for their 'honest' admission over #IranPlaneCrash. FYI, 'honest' & 'Iran regime' are words that do not go together. Recall, first Iran vehemently denied they had anything to do & blamed Trump. Only conceded given mounting intelligence & threat of action."
The Ukrainian Airlines' Flight 752 had crashed on Wednesday after it had taken off from Tehran's airport, hours after Iran fired missiles at Iraqi military bases that housed US troops.
The action was in retaliation to a drone strike in Baghdad that killed Iranian commander Qasem Soleimani.
( With inputs from IANS )