SEOUL — The U.S. ambassador to South Korean was recovering late Thursday after he was attacked here by a nationalist in a rare spasm of anti-American violence that runs contrary to mainstream support in this country for close ties between Seoul and Washington.

The activist slashed the ambassador, Mark Lippert, with a 10-inch kitchen knife as the envoy was seated, preparing to give a Thursday morning breakfast address about the U.S.-South Korean alliance. The attacker, a bearded man dressed in a traditional Korean outfit, screamed calls for reunification between North and South Korea.