UNITED NATIONS — The next test of the United Nations will come tomorrow, when South Korea, with Washington’s backing, plans to ask the Security Council to take up the sinking in March of its ship, which according to Seoul investigators was attacked by a North Korean torpedo. The council is not expected to officially convene on the matter before next week.

Even then, the world body may be out of whatever is the diplomatic equivalent of ammo. Envoys and aides here say they’re stymied as to what the Security Council can do beyond issuing a rebuke of North Korea for its recent deadly attack against a South Korean vessel. As one diplomat from the region told me yesterday, “there’s little we can do” to further tighten the existing sanctions that the Security Council has already imposed on North Korea.

Other diplomats familiar with the council’s dynamics said that the council may issue a reprimand to North Korea, but the severity of the wording of the rebuke largely depends on China. Beijing has enabled the Pyongyang regime and recently hosted the normally reclusive Kim Jong-il for talks with top Chinese officials.

It’s unclear even whether China would allow the council to endorse the findings of a two-month South Korean investigation into the March 26 explosion of a warship in a disputed border area between the two Korea. According to reports from Seoul, local officials have briefed foreign diplomats in the last two days on the findings of the South Korean forensic probe, which was aided by American experts.

Among other discoveries unearthed by the investigation were fragments of a North Korean torpedo. One of the fragments carried a discernable serial number. According to the reports, the investigation’s findings show that the ship’s sinking, which killed the 46-member crew, was committed by the North Korean military.

The Security Council responded last year to a nuclear test conducted by North Korea on May 25 by issuing a resolution that according to the American ambassador here, Susan Rice, established the strongest sanctions regime on any country to date.

Several diplomats, who declined to be identified while the closed-door negotiations among council members are ongoing, said they doubted the council would further tighten sanctions at this stage. The negotiations over a draft resolution proposing further sanctions on Iran, which was introduced by the five permanent council members Tuesday, may complicate the North Korea discussions at the council as well, they added.