The top U.S. diplomat for East Asia called a failed North Korean rocket launch a security threat Monday. He declined to talk about possible punishing sanctions but said the world would unite to stop more provocations.

Three days after the long-range rocket broke into pieces over the Yellow Sea shortly after liftoff, generating international condemnation and costing North Korea a food aid-for-nuclear freeze deal with Washington, Assistant Secretary of State Kurt Campbell was carefully diplomatic in comments to reporters at Seoul's Foreign Ministry.

Tough sanctions haven't stopped North Korea in recent years from conducting two nuclear tests and four long-range rocket launches that Washington calls covers for tests of missile technology designed to reach the United States.

But Campbell, speaking after meetings with senior South Korean officials, said the United States and its allies are determined "to meet provocations with a very clear and firm coordination. I believe that that is the best possible response to what we have seen from North Korea."

Asked if that response included strong new U.N. sanctions, Campbell said consultations are under way at the United Nations and he wanted "to let that process play out."

The U.N. Security Council has so far denounced the launch as a violation of resolutions that prohibit North Korea from developing its nuclear and missile programs. The council imposed sanctions on North Korea after its first nuclear test in 2006 and stepped up sanctions after its second in 2009.

"Even though the launch was a failure, it was a provocative action that threatens international security" as well as violating U.N. resolutions and other North Korean obligations, Campbell said. "The international community is united in its strong determination to discourage any further provocation."

North Korea said the rocket was meant to launch a peaceful observational satellite into orbit as part of celebrations marking the 100th anniversary of the birth of the country's founder, the late Kim Il Sung.

A day before Campbell's visit to Seoul, North Korea's new leader, Kim Jong Un, addressed his nation and the world for the first time, giving a speech during the centennial festivities in which he vowed to place top priority on his impoverished country's military.

Pyongyang also unveiled a new long-range missile, although it's still not clear how powerful or significant the addition to the North Korean arsenal is. Some analysts suggested it might have been a dummy designed to dupe outside observers.

Campbell wouldn't talk about worries that North Korea may now feel itself under pressure to make up for the rocket failure with a nuclear test — a pattern that happened in 2006 and 2009. South Korean intelligence officials have said that recent satellite images show North Korea is digging a new tunnel in what appears to be preparation for a third nuclear test at the site of its two past tests.