"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 the test today 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. (Read: North Korea carries out underground nuclear test)
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".
The UN Security Council was scheduled to meet in emergency session later today 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.