South Korea balked Monday at Washington's demand that it fully join a U.S.-led effort to intercept North Korean ships suspected of carrying supplies for the North's nuclear and missile weapons programs.

The South insisted that it was already doing enough to stem possible weapons proliferation from North Korea -- which detonated a nuclear bomb on Oct. 9 -- and announced no new measures to sanction the North under a U.N. Security Council resolution condemning the test.

The decision underscored Seoul's reluctance to anger Pyongyang and complicated efforts to resolve the standoff over the North's nuclear program now that the communist regime has agreed to return to long-stalled international nuclear disarmament talks.

"It's basically not necessary to take (new) measures," Park In-kook, a deputy foreign minister, said at a news briefing, citing "the unique circumstances" on the Korean peninsula as motivating the South's decision not to fully join the U.S.-led Proliferation Security Initiative, or PSI.

Seoul has joined the initiative aimed largely at stopping North Korean weapons traffic at sea, which has gained new impetus since the North's nuclear test, only as an observer out of concern that its stopping and searching North Korean ships could lead to armed clashes with North Korea.

Monitoring North Korean shipping would be much more difficult without South Korea, because countries in the initiative can only conduct searches within the territorial waters of participating countries. Ships on the high seas have right of free passage under international law.

South Korea is to submit a report Monday on how it would carry out the unanimously adopted Security Council resolution to a U.N. committee charged with overseeing the sanctions on North Korea.

The resolution bans the sale of major arms to North Korea and calls for the inspection of cargo entering and leaving the country. It also calls for the freezing of assets of businesses supplying the North's nuclear and ballistic weapons programs, as well as restrictions on sales of luxury goods and travel bans on Pyongyang officials.

South Korea has rejected criticism that it is too soft on North Korea, citing as an example of its toughness its suspension of humanitarian aid to the impoverished North after it test-fired a series of missiles over international objections.

The South insists it could inspect North Korean ships under an inter-Korean agreement but it has never done so despite allowing dozens of Northern vessels to transit its waters.

On Monday, Lee Kwan-se, an official of the Unification Ministry that deals with reconciliation with North Korea, said the South would continue a hold on regular humanitarian aid to the North.

Lee also said South Korea will suspend subsidies it pays for a tourism program at the North's Diamond Mountain resort, and also keep on hold an expansion plan for an inter-Korean industrial park in the North's border city of Kaesong.

The two projects, considered key symbols of inter-Korean reconciliation, are a major source of hard currency for the cash-strapped North and have been under fire over concern that they may fund the North's missile and nuclear programs.

Seoul's measures against the projects are not expected to affect them seriously because the subsidies for the tour project are believed to be rather small and the industrial zone expansion plan is already in limbo due to the North's provocations.

The two Koreas are still technically at war as the 1950-53 Korean War ended in a truce, not a peace treaty. But their relations have warmed since the first, and only, summit of their leaders in 2000, with Seoul pursuing engagement rather than confrontation under the so-called sunshine policy.