"With God's help, and without paying attention to propaganda, Iran's nuclear course should continue firmly and seriously ... Pressures, sanctions and assassinations will bear no fruit. No obstacles can stop Iran's nuclear work."
Khamenei was speaking on state television shortly after the U.N. nuclear watchdog declared a collapse in talks with Iran aimed at getting it to address suspicions that it is covertly seeking nuclear weapons capability.
The Islamic Republic denies this, saying its program to enrich uranium for nuclear fuel is for peaceful energy only.
But Iran's refusal to curb sensitive atomic activities with both civilian and military purposes, and its track record of secrecy and restricting U.N. inspections, have drawn increasingly harsh U.N. and separate U.S. and European sanctions, now targeting its economically vital oil exports.
Several Iranian nuclear scientists have been killed in bombings over the past two years, attacks that Tehran has blamed on arch-adversary Israel. The Jewish state has not commented.
The United States and Israel have not ruled out resorting to military action against Iran if they conclude that diplomacy and sanctions will not stop it from developing a nuclear warhead.
(Reporting by Parisa Hafezi; Editing by Mark Heinrich)