UNITED NATIONS (Reuters) - The U.N. Security Council is united on North Korea's nuclear arms program and will undoubtedly approve tough measures against Pyongyang if it carries out a new atomic test as expected, South Korean U.N. Ambassador Kim Sook said on Monday.

"The North Korean nuclear test seems to be imminent," Kim, who is president of the Security Council this month, told reporters. "Obviously there are very busy activities going on at the (North Korean) nuclear test site, and everybody's watching."

"Everybody is unified and they are firm and resolute," he said. "I would expect very firm and strong measures to be taken ... once they go ahead with such provocation."

(Reporting By Louis Charbonneau; Editing by Sandra Maler)