In an interview on state-run television, Bashar Assad said there will be repercussions to any country interfering in Syria's affairs.
His remarks came just days after the United States and its European allies called for him to step down, and hours after a diplomat said Assad's regime was "scrubbing blood off the streets" ahead of a U.N. visit.
Assad repeated plans to introduce reforms. The opposition rejected Assad's remarks, saying they have lost confidence in his promises of reform while his forces open fire on peaceful protesters.
The uprising against the Assad family's 40-year rule erupted in March.
Human rights groups said more than 2,000 anti-regime demonstrators have been killed in Assad's brutal crackdown, including two this weekend.
The Associated Press contributed to this report.