South Korean authorities maintained high alert on Wednesday over concerns of possible further provocations from Pyongyang including additional nuclear and missile tests.

“North Korea appears likely to carry out further provocations using the UN Security Council’s strong measures as an excuse,” an official at the Ministry of Foreign Affairs and Trade was quoted as saying by a local news agency. He added that it is unclear when the alert triggered by North Korea’s actions would be eased.

Such speculation is supported by an official statement Pyongyang released eight hours after Tuesday’s test, in which it said that the test was a “primary response measure” against the U.S. hostilities towards North Korea.

The statement also said that additional steps could be taken “depending on U.S.’s stance” and threatened a “merciless retaliatory strike” against punitive measures the international community imposes on Pyongyang.

Previous to the test, Seoul’s military has been preparing for a scenario where Pyongyang conducts two or more nuclear tests in succession as it tries to diversify its nuclear weapons technology.

Local authorities also have not ruled out the possibility of a plutonium device detonation or a nuclear fusion test being carried out.

“In addition to further nuclear tests, North Korea could test fire an intercontinental ballistic missile from a mobile launch system, or declare that nuclear warheads will be put into active service,” National Intelligence Service Director Won Sei-hoon told the parliamentary intelligence committee on Tuesday. The NIS chief also said that the authorities are prepared against the possibility of Pyongyang carrying out military provocations in order to influence discussions for sanctions and to bring China back on its side.

Regarding Pyongyang’s nuclear capabilities, Won said that the North appears to have secured the necessary technologies and long range ballistic missile capabilities, but that the country remains short of being able to make nuclear warheads small and light enough for missiles.

He added that South Korea should continue the efforts to induce Pyongyang to abandon nuclear ambitions, while remaining under the U.S. nuclear umbrella.

According to Seoul’s Ministry of National Defense, the North prepared two underground shafts at the site in Punggye-ri, North Hamgyeong Province before Tuesday’s test.

“We think that the second shaft stands ready, so another test could be conducted at any time even if no movements are detected at the site,” Defense Ministry spokesman Kim Min-seok said at a press briefing on Wednesday.

“In general, it is thought that surveillance (for signs for further tests) should be maintained for between 48 and 72 hours.”

By Choi He-suk  (cheesuk@heraldcorp.com)