"The pipeline carrying Kazakh oil to Azerbaijan will be constructed on the seabed and could pose threats to the ecological system of the Caspian Sea," Iran's Deputy Oil Minister for International Affairs Hossein Noghrekar Shirazi said.
"Iran is against any plan that could disturb the ecological balance of the Caspian Sea," press tv quoted Shirazi as saying on Saturday.
Shirazi announced that Russia also opposes the construction of the pipeline.
The United States is said to have succeeded in getting Kazakhstan to sign a memorandum that might assist it to send oil across the Caspian Sea to Baku-Tbilisi-Ceyhan (BTC) pipeline.
Iran and Russia strongly oppose the project on environmental and political grounds.