South Korea on Thursday warned North Korea of "grave consequences" and new international sanctions if the North goes ahead with a nuclear test, with Seoul officials assuming that Pyongyang is technically ready to conduct a third nuclear test.

Officials and analysts believe that the North may soon set off a nuclear device following its failed launch of a long-range rocket on April 13. Pyongyang's two previous rocket launches in 2006 and 2009 were followed by nuclear tests.

The U.N. Security Council tightened sanctions against North Korea over the failed launch, strongly condemning it as a violation of U.N. resolutions that ban the North from testing ballistic missile technology and warning of additional actions if Pyongyang conducts another missile or nuclear test.

"If North Korea goes ahead with a nuclear test, it will be a clear breach of the April 19 presidential statement by the U.N.

Security Council and therefore the Security Council will have to take new actions," foreign ministry spokesman Cho Byung-jae said.

Should North Korea go ahead with a nuclear test, Cho said it "will bring about grave consequences that do not help North Korea at all."

South Korea has stepped up monitoring of activities at the North's nuclear test site, but it is difficult to figure out whether a test is imminent, Cho said.

In a telephone interview with Yonhap News Agency on Wednesday, a senior government official said that North Korea appears to have finished preparations and is "technically ready" to set off a nuclear device.

Also on Thursday, defense ministry spokesman Kim Min-seok echoed a similar view, saying the North's nuclear test is just a matter of time and Pyongyang is waiting to make a "political choice."

"Our judgment is that North Korea can conduct a nuclear test at any time and a political choice is left" before testing, Kim said. 

South Korea, the U.S. and Japan have pressed North Korea to back down on a nuclear test, but the North vowed to boost its nuclear deterrent as long as the U.S. sticks to what it calls a "hostile policy" against the communist regime. 

"We had access to nuclear deterrence for self-defense because of the hostile policy of the U.S. to stifle the DPRK (North Korea) by force and we will expand and bolster it nonstop as long as this hostile policy goes on," the North's foreign ministry said in a report Tuesday by the Korean Central News Agency.