"It is natural that we politically back those Palestinian groups that are fighting for the rights of their people," Karroubi told reporters when asked about his stance on Hamas.
Iran is a staunch supporter of Hamas, which won Palestinian elections in 2006 but whose government was boycotted by the West. It seized control of the Gaza Strip in 2007, effectively cleaving the Palestinian territories into two separately ruled entities.
In 2006, Tehran pledged millions of dollars in aid to the Hamas government, but the Islamic republic has always insisted its support for Palestinian militant groups does not extend to arming or training fighters.
Iran does not recognised Israel and President Mahmoud Ahmadinejad, who is seeking a second term in office, has drawn global outrage for a number of anti-Israel tirades.
Karroubi, one of three candidates challenging Ahmadinejad in the June 12 election, has vowed to adopt a policy of detente with the international community.
He has ruled out pulling out of the race in order to increase the chances of reformists but said he would support moderate former prime minister Mir Hossein Mousavi if the election goes to a second round.
"I will definitely support Mr Mousavi if he remains in the second round against Mr Ahmadinejad."
