LONDON-Insurers last week still were trying to assess the likely costs of the oil spillage from the Russian tanker Nakhodka after the vessel broke in two soon after the start of the year during severe storms in the Sea of Japan.

The biggest potential insurance claims are likely to fall on the vessel's liability insurers, The United Kingdom Mutual Steam Ship Assurance Assn. (Bermuda) Ltd., known as the UK P&I Club. The club insures the vessel's owners, the Russian shipping company Prisco Traffic Ltd., for the costs of pollution damage, cleanup and fines.

However, Graham Daines, who heads the London-based claims department at the club's managers, Thos. R Miller & Sons (Bermuda), said last week that those costs still were being assessed.

More than two-thirds of the 19,000 metric tons of oil had already leaked into the sea from the two sections of the ship, and the cleanup operation is expected to take several weeks. Initial hopes that the crude would be dispersed at sea using detergents before it reached shore were dashed by continued bad weather and rough seas.

Reports from Japan's Maritime Safety Agency said that by last Tuesday the oil had reached the shore near the fishing port of Mikuni, in Japan's Fukui prefecture. At least 188 miles of shoreline were affected, which included important fish farms, fishing ports and tourist resorts. An improvement in the weather Wednesday enabled government workers and local volunteers to start cleanup work, but the P&I club still was awaiting word of how successful these measures would prove.

A statement Wednesday from the London-based International Tanker Owners Pollution Federation said that Prisco has contracted the Japan Disaster Prevention Center to undertake cleanup operations using its own equipment and manpower resources and those of a network of local contractors.

The owners have also obtained equipment from the Petroleum Assn. of Japan, which includes containment booms, skimmers, vacuum units and temporary storage units. The JDPC and its contractors are using this equipment to collect floating oil and to protect sensitive areas.

The tanker, which was built in 1970, was insured for only $1.8 million, the hull value, with the Russian insurer Zascita, and 85% reinsured with Black Sea & Baltic General Insurance Co. Ltd., a London-based unit of Russian insurer Ingosstrakh.

The Nakhodka was on a voyage from China to Russia at the time of the incident. The 31 crew members were saved, but the captain was reported missing.