By Elise Labott and Barbara Starr

North Korea's nuclear test Tuesday set off a diplomatic scramble for America's new secretary of state as the U.S. national security community began working with other countries to try to determine what North Korea truly achieved.

The test was was not a total surprise, senior administration officials said. North Korea warned the United States and China on Monday that it would be undertaking a nuclear test, two senior administration officials told CNN. The warning came in the form of a message through the "NY channel," which is the U.S. mission to the United Nations, North Korea's typical method for passing messages to the United States. The warning was not specific on timing, but the officials said Washington took it to mean the test could happen at any moment.

After the test was detected late Monday night, Secretary of State John Kerry spoke with South Korea's foreign minister. He's also expected to talk with the foreign ministers for China, Japan and Russia. The United States began coordinating its own response with inter-agency calls between Washington and Seoul, Tokyo, Moscow and Beijing. U.S. Ambassador to South Korea Sung Kim and Gen. James Thurman, commander of the US-Republic of Korea Combined Forces Command, met with the South Korean defense minister.

The U.S. intelligence community and military began the process of assessing the test and North Korea's claims and by morning concluded an underground nuclear test had probably been conducted.



"The explosion yield was approximately several kilotons. Analysis of the event continues," said a statement released Tuesday by the Office of the Director of National Intelligence.

Nuclear monitoring by the Comprehensive Test Ban Treaty Organization will calculate what was released by the explosion. The United States will likely try to get additional readings from aircraft equipped with sensors and satellites and attempt to match those with seismic data to calculate what the explosion was.

Officials said it will be several days before Washington has a handle on the actual size of the explosion, payload and the success, or lack thereof, of the test. Bottom line: Successful or not, these tests do help North Korea learn and continue to perfect the technology.

One senior administration official says one of the the things the United States will be trying to determine is whether the North Koreans tested a uranium weapon for the first time. The past two tests were plutonium.

Washington and its allies also will be trying to figure out what type of technology and engineering was used to pull off the test and what, if any, help and materials North Korea may have received from other countries like Iran.

In addition to the analysis of data, the United States is currently analyzing North Korea's statement. At first glance, the mention of a "miniaturized" nuclear weapon is cause for concern, because that would mean the North Koreans may have mastered the technology to fit a nuclear weapon on a long-range missile that could possibly reach the United States. North Korea successfully demonstrated it could launch a long-range missile through its rocket launch last December, but questions remain about its ability to aim it toward a long-range target or even repeat the launch success.

United Nations Security Council members are meeting Tuesday morning about the North Korean test and officials say the American playbook will be pretty familiar. The officials all said Washington will seek a "tough, swift reaction" from the Security Council. Obviously, what comes out of the council will depend on what China and Russia will agree to sign on to. But one official said "there is a pretty strong commitment to go with a seriousness of purpose" to the United Nations.

Officials said the Obama administration is also looking at what other kinds of sanctions can be slapped on North Korea, which is already probably the most-sanctioned entity in the world. They are looking at the playbook for Iranian sanctions - shipping, insurance, more sanctions against financial transactions, designations of individuals.

"There is plenty to do," another senior official said.