"The bombings on Gaza must stop immediately ... the path of oil and food exports to the Zionist regime should be stopped," Khamenei told a group of students in Tehran.
Israel has vowed to wipe out Tehran-backed Hamas, which rules Gaza, in retaliation for an October 07 attack that killed 1,400 people and saw hundreds taken hostage.
Israel has launched an unprecedented bombardment of Gaza and imposed a siege of the enclave. Palestinian authorities say more than 8,000 people have been killed.
Iran's clerical rulers have warned Israel of an escalation if it failed to end aggressions against Palestinians, with authorities indicating Tehran-backed proxies in the Middle East were ready to act.
Backing the Palestinian cause has been a political pillar of the Islamic Republic since the 1979 Islamic Revolution and a way in which its Shi'ite Muslim theocracy has fashioned Iran as a leader in the Muslim world.
Khamenei, Iran's top authority, said the United States was complicit in Israel's "recent crimes against Palestinians".
"The Islamic world should not forget that in the crucial issue of Gaza, those standing against the oppressed Palestinian nation were the United States, France and Britain," Khamenei said to chants of "Death to Israel" and "Death to America".
Israel, which Tehran refuses to recognize, has long accused Iran's clerical rulers of stoking violence by supplying arms to Hamas. Tehran says it gives moral and financial support to the group, which controls the Gaza Strip.
"One of the shameless acts of the West is accusing Palestinian fighters of terrorism," Khamenei said.
