The arrest by Danish special forces of four suspected pirates off West Africa could leave Denmark with a legal headache, a policy expert believes.

Frogmen from the frigate Esbern Snare opened fire on a suspicious boat in waters south of Nigeria on Wednesday night, killing four men and injuring another.

Danish police later said on Twitter that the four survivors appeared in absentia at Copenhagen City Court on Thursday, when they were remanded in custody for 13 days.