A spokesman at the Dutch foreign ministry said: "We are aware of an incident and there was some Dutch involvement, but cannot confirm whether there were any Dutch casualties."
 
One security source said armed men in two boats approached the ferry near Bonny Island in the southern delta.
 
He said the ferry was transferring crew members for an unidentified company.
 
He also said two Nigerian naval officers armed with automatic weapons were aboard the boat to provide security for the oil workers.
 
It was unclear who opened fire first.
 
Tribal deaths raise tension
 
In a separate incident, a subsidiary of Royal Dutch Shell evacuated two oil installations elsewhere in Rivers State after fighting claimed the lives of 12 local chiefs.
 
Bisi Ojediran, a spokesman for the company, said that the move was precautionary and would not affect production.
 
Last month oil companies evacuated hundreds of workers' families from the eastern delta after two car bombings in company compounds.
 
Nigeria's delta region remains impoverished, despite producing tens of billions of dollars in oil revenues every year.
 
Attacks and kidnappings targeting foreign workers have become common and are often carried out by groups demanding the region receive a greater investment of the country's oil wealth.