Reports from Somalia say four Ukrainians are being held by the local authorities in the north-eastern Bosasso region.
They are said to have been arrested by local fishermen but it not clear why.
Attempts are being made to negotiate their release.
Last week the crew of a Kenyan-registered ship were fined more than five-hundred-thousand dollars by court in the region for illegal fishing.
They are still in custody.
From the newsroom of the BBC World Service