In his most outspoken comments yet, Mr Ahmadinejad told CNN: "Nobody has the right to kill others, neither the government nor its opponents."
He said Iran would encourage all sides to reach an understanding, but warned the US not to intervene in Syria.
Syria has close ties with Iran, which suppressed its own protests in 2009.
Iran has also put down or prevented about a dozen protests since the wave of anti-government uprisings in the Middle East began earlier this year.
"We are going to make greater efforts to encourage both the government of Syria and the other side, all parties, to reach an understanding," Mr Ahmadinejad said in the interview with CNN.
He warned against any outside intervention in Syria, in particular by the US.
"The positions of the United States are not going to help. They have never helped," he said.
Iran has been muted in its criticisms of Syria, its most important ally in the region.
However in September, Mr Ahmadinejad spoke of "needed reforms", while the Iranian foreign minister called on Syrian President Bashar al-Assad to "be accountable to his people's legitimate demands".
Shia Iran is regarded with suspicion by its Sunni Arab neighbours. Syria's government is dominated by the Alawite sect, an offshoot of Shia Islam.

If Prime Minister Theresa May does not appear to have had a particularly good night, Ruth Davidson, the Tory leader in Scotland, certainly has.
The Scottish Tories have seen double-digit increases in their vote in almost every seat. They've more than doubled their returns in some places where the party was previously unelectable.
First they knocked off Angus Robertson, the SNP's deputy leader and top dog at Westminster.
They had called that their "Portillo moment" - but their best was yet to come, as they dethroned Alex Salmond in Gordon. They even overturned the fourth-safest seat in the country, in Banff and Buchan.
Other Scottish Tory gains include:
It's been a long time since the Tories have had a night like this in Scotland.
In four elections in a row, they returned a single MP. The year before that, 1997, they got zero. You have to go back to 1992 for the last time they won more than a single seat in Scotland.
They ended up with 13 - the party's best showing north of the border since 1983.
What this Tory resurgence means for the rest of the UK in terms of parliamentary calculations is unclear. The margins are razor-thin, so every seat is precious.
But the Tories are resolute in what they believe it means for Scotland.
"Indyref2 is dead," Ms Davidson says. The prospect of a second independence referendum has been at the core of the Tory campaign, and indeed their entire revival in Scotland.
The SNP have a majority of seats, and thus a "triple lock" mandate for that referendum, according to their manifesto - but it certainly seems a more distant prospect than ever.
And while Mrs May's future might now be in doubt, Ms Davidson's star is burning more brightly than ever.
Iran's President Mahmoud Ahmadinejad has criticised the killings in its ally Syria sparked by the government's violent crackdown on dissent.
[NEXT_CONCEPT]
Election night has turned out to be a night of contrasting fortunes for the Conservative and Unionist Party.