A Cosco boxship was detained in Indonesia for nearly a month over "security concerns", the Chinese company has revealed.

The Bisnis website cited Cosco Shipping Lines Indonesia's deputy managing director Hendra Kusuma as saying the 4,250-teu Seaspan Fraser (built 2010) was finally released on Thursday.

It was carrying 1,700 boxes of exports from East Java and is chartered from Seaspan for a Surabaya to Singapore feeder service.