he top U.S. envoy for arms control says North Korea, Iran and Syria are among the worst proliferators of weapons of mass destruction.

U.S. Undersecretary of State John Bolton told reporters in Tokyo that the three countries are states of proliferation concern. He called for closer monitoring of shipments to and from such countries.

Mr. Bolton is visiting Tokyo to observe multinational exercises, held under the U.S.-led Proliferation Security Initiative, aimed at training troops from several countries to intercept weapons of mass destruction at sea.

The United States says the drills send a message that the world does not tolerate anyone who tries to traffic weapons of mass destruction.