SEOUL, Feb 12, 2013 (AFP) - North Korea said its nuclear test Tuesday was only a "first" step and warned of stronger action if it was faced with tougher sanctions as a result.

"The latest nuclear test was only the first action, with which we exercised as much self-restraint as possible," the foreign ministry said in a statement carried by the country's official news agency.

"If the US further complicates the situation with continued hostility, we will be left with no choice but to take even stronger second or third rounds of action," it said without elaborating.

The statement came just hours after South Korea's spy agency chief warned Pyongyang might carry out another nuclear test or ballistic missile launch in the coming days or weeks.

North Korea said Tuesday's test was directly targeted at the United States. It accuses Washington of inciting global condemnation of its nuclear programme and of leading the sanctions charge in the UN Security Council.

The ministry statement said any measure like forced ship inspections or a sea blockade would be considered an "act or war" and trigger "merciless retaliations".

UN to meet in emergency session

The UN Security Council was scheduled to meet in emergency session later Tuesday to discuss the international response to the latest test.

"The US should make a choice between taking the path of easing tension... or continuing on its current, wrong path of pursuing anti-DPRK (North Korea) policies and further escalating tensions," the ministry said.

While the Russian Foreign Minister Sergei Lavrov said Tuesday he expected the UN Security Council to agree on "an adequate response" to North Korea's controversial nuclear test.

"I know that the UN Security Council is to be discussing this issue in the next few hours and (will) make relevant agreements and decisions on that," he said via a translator on a visit to Pretoria.

The Tanaiste Eamon Gilmore has also condemned the act and has called on North Korea to live up to its international obligations.