SEOUL, South Korea With North Korea's announced plan to launch a rocket as early as this weekend, the 16-year-old nuclear crisis on the Korean peninsula confronts a third U.S. president, looming large on Barack Obama's agenda as he met his South Korean counterpart today at the G-20 summit in London.

Once again, harsh language is rumbling out of Pyongyang, with its Foreign Ministry spokesman warning that "even a single word critical of the launch" from the Security Council will be regarded as a "blatant hostile act."

North Korea says the rocket to be launched between April 4 and 8 will carry a satellite into space but the United States, South Korea and Japan have warned that any launch whether of a satellite or a long-range missile would violate a United Nations Security Council Resolution prohibiting Pyongyang from ballistic activity, and could draw sanctions.

Obama and South Korean President Lee Myung-bak met today on the sidelines of a 20-nation summit on the economic crisis, agreeing on the need for a "stern, united" international response if North Korea goes ahead with the launch.

Obama told Chinese President Hu Jintao on Wednesday that the U.S. would consider the launch provocative and that the U.S. would seek punishment at the United Nations in response.

North Korea claims sanctions would violate the spirit of disarmament agreements and would nullify all existing measures to denuclearize the Korean peninsula.

That would in effect reverse a process that has moved in fits and starts, holding back the threat of war but never achieving much detente.

The latest developments, coupled with North Korea's arrest and threatened indictment of two U.S. journalists, are in sharp contrast to June 2008, when North Korea made a seemingly promising move toward disarmament by blowing up a cooling reactor at its Yongbyon nuclear complex.

Kim Jong Il's regime routinely backtracks on agreements, refuses to abide by international rules and wields its nuclear program as a weapon to win concessions from Washington or Seoul, says Koreas expert Peter M. Beck.

"History has shown them that the more provocative they are, the more attention they get. The more attention they get, the more they're offered," he said from Washington.