SEOUL, South Korea — North Korea warned South Korea against joining international sanctions, saying yesterday that its neighbor would “pay a high price” if it joins the American-led drive to punish the reclusive communist nation for its nuclear test.

The statement from the North’s Committee for the Peaceful Reunification of the Fatherland came as South Korea struggles to determine how it should enforce the U.N. sanctions, including whether to help interdict North Korean cargo ships suspected of transporting materials for unconventional weapons.

“If the South Korean authorities end up joining U.S.-led moves to sanction and stifle [the North], we will regard it as a declaration of confrontation against its own people … and take corresponding measures,” the North’s Committee for the Peaceful Reunification of the Fatherland said in a statement.

The U.N. Security Council unanimously adopted a sanctions resolution five days after the North’s October 9 test, and a South Korean task force met this week to determine how the country should address the measures, including what to do about joint economic projects with the North.

South Korea’s participation in the sanctioning the North is important because the country is one of the main aid providers to the impoverished communist nation, along with China.

But both countries have been reluctant to impose stern measures against their volatile neighbor. North Korea’s closest ally, China, voted for the U.N. resolution but is concerned that excessive measures could worsen the situation. South Korea has expressed similar concerns, although there was no immediate response to yesterday’s statement from North Korea.

“If North-South relations collapse due to reckless and imprudent sanctions against us the South Korean authorities will be fully responsible for it and will have to pay a high price,” the statement, carried by the North’s official Korean Central News Agency, said.

A top American diplomat said North Korea’s test has brought China and America closer together and that both countries want a unified response.

“China has been in a very important relationship with us for many years, and at no time did we feel any closer together with China than we felt in the wake of the North Korea provocation,” Assistant Secretary of State Christopher Hill told reporters during a meeting of Pacific leaders in Fiji.