North Korea has said it is now capable of mounting nuclear warheads on ballistic rockets after conducting what it said was a successful nuclear test.
World leaders were quick to condemn the development.
South Korean President Park Geun-hye said North Korean leader Kim Jong-un showed "maniacal recklessness" by testing the nuclear device.
Park and US President Barack Obama talked by telephone after the test was conducted, South Korea's Blue House said.
READ MORE: North Korea hails 'successful' nuclear test
South Korea's presidential office said Obama had indicated he would take every action under a mutual defence treaty with the South, including providing its nuclear umbrella, to protect Seoul from any North Korean threat.
The presidential office also said the two leaders agreed to use every available means to pressure the North to abandon its nuclear programme, including adopting a new UN Security Council resolution.
South Korea's vice finance minister said the government would closely monitor financial markets and guard against geopolitical risks linked to North Korean actions, and promised to take measures to stabilise markets if needed.
The government expects North Korea's nuclear test to have limited negative impact on the economy, Choi Sang-mok said, in a meeting with officials from the central bank and regulator that was urgently scheduled in Seoul.
China's foreign ministry said on Friday that it was "resolutely opposed" to North Korea's latest nuclear test and strongly urged Pyongyang not to take any action that would worsen the situation.
China, the North's main ally, would stick to its aim of de-nuclearising the Korean peninsula and uphold the six-party talks process to resolve the issue, the ministry added in a short statement.
The international community should exercise restraint after the test because it is in nobody's interest for there to be chaos or war on the Korean peninsula, China's official Xinhua news agency said.
North Korea's nuclear test was "not wise", but South Korea's decision to deploy an advanced US anti-missile system has also seriously damaged regional strategic balance, Xinhua said in a commentary, in China's first official reaction to the test.
Obama said any provocative actions by North Korea would have "serious consequences".
The US president had been briefed on board Air Force One by National Security Adviser Susan Rice about the nuclear test, White House press secretary Josh Earnest said.
Obama reiterated an unbreakable US commitment to the security of its allies in Asia and around the world, Earnest said.
US Assistant Secretary of State Daniel Russel said he was committed to working closely with Japan, China, Russia and South Korea to put pressure on North Korea.
"The fact is, there is an accelerated pattern of very serious provocation that violates international law and threatens regional stability," Russel told reporters before meeting Japanese Foreign Minister Fumio Kishida.
Japanese Foreign Minister Fumio Kishida said that Tokyo had protested to North Korea.
A nuclear test would breach UN resolutions and pose a grave challenge to global nuclear non-proliferation efforts, Kishida told reporters.
"France vigorously condemns the new nuclear test that was conducted last night by North Korea and calls the United Nations' security council to take up this violation of its resolutions," the French presidency said in a statement.
The UN atomic agency, IAEA, in a statement called the test "deeply troubling".
"This is in clear violation of numerous UN Security Council resolutions and in complete disregard of the repeated demands of the international community. It is a deeply troubling and regrettable act.
"In Resolution 2270, the Security Council condemned in the strongest terms the DPRK’s previous nuclear test conducted on 6 January this year.
"The Security Council also reaffirmed its decisions that the DPRK shall not conduct any further nuclear tests and shall abandon all nuclear weapons and existing nuclear programmes in a complete, verifiable and irreversible manner, and immediately cease all related activities."
IAEA Director General Yukiya Amano said he "strongly urges" the DPRK to fully implement all relevant resolutions of the UN Security Council and the IAEA.
"The Agency continues to closely follow the DPRK nuclear issue. We remain ready to contribute to its peaceful resolution by resuming our verification activities in the country once a political agreement is reached among countries concerned," Amano said.