Somali pirates have released a Singapore-owned hijacked chemical tanker, but continue to hold four South Korean sailors as hostages.

The Singapore-based firm Glory Ship Management, which owns the ship, said Thursday that the pirates had promised to release all 25 crew members, but instead took the remaining four “ashore at the last moment.”

The company says the released crew members are in good health and that it is working to secure the release of the remaining hostages.

Pirates seized the tanker off the east African coast in April while it was travelling from Indonesia to Kenya.

The crew consisted of 13 Indonesians, five Chinese and three Burmese, in addition to the South Koreans.

South Korea's Foreign Ministry says it is also working to have the remaining hostages released, but reiterated its position not to negotiate with pirates.

In January, South Korean military commandos rescued the crew of a hijacked freighter in the Arabian Sea, killing eight Somali pirates.

Five other pirates were captured in the raid and given long prison sentences.