"What would happen after that? Humanitarian disaster, a vast number of refugees, Iran's wish to take revenge - and not only upon Israel, to be honest, but upon other countries as well."
"But my Israeli colleagues told me they were not planning to act in this way, and I trust them," he said.
Israel accuses Iran of developing nuclear weapons and has suggested it could intervene by force if Tehran continues to refuse to suspend uranium enrichment.
Iran has repeatedly insisted that its nuclear programme is for meeting civilian energy needs rather than producing an atomic warhead.
In the interview, Medvedev also confirmed that Binyamin Netanyahu, the Israeli prime minister, made a secret visit to Moscow earlier this month that included a meeting with the Russian president.
"Iran must co-operate with the IAEA, this is absolutely obvious, if it wishes to develop its nuclear dimension"
Netanyahu's office said he had been visiting a secret security facility on September 7, but there was an angry response in the Israeli media after it emerged that he may have travelled to Russia instead.
There was widespread speculation that he had gone to Russia to pressure Moscow not to deliver a number of S-300 air-defence missiles to Iran, but Medvedev did not reveal the purpose of the meeting.
Two years ago, Moscow signed a contract to sell S-300s to Iran, a move that has been criticised by Israel.
In the interview, Medvedev acknowledged Israel's concerns but said that "any supplies of any weapons, especially defensive weapons, cannot increase tension; on the contrary, they should ease it."
Russia has cultivated close co-operation with Iran, including building the Bushehr nuclear power plant, which critics say is a key element of Iranian attempts to develop nuclear weapons.
However, it has also been critical of Iran's failure to co-operate with the International Atomic Energy Agency (IAEA) as it seeks to determine if Iran is pursuing nuclear arms.
Medvedev renewed this criticism in the CNN interview.
"Iran must co-operate with the IAEA, this is absolutely obvious, if it wishes to develop its nuclear dimension, its nuclear energy programme.
"This is a duty and not a matter of choice," he said.