Nigeria's president has ordered the country's armed forces to boost security in the southern Niger Delta region following an attack by militants on an offshore oil field.

In a statement Friday, President Umaru Yar'Adua said his government would take "all necessary action" against those who attacked the Royal Dutch Shell oil installation on Thursday.

The region's main militant group, the Movement for the Emancipation of the Niger Delta, claimed responsibility for the rare offshore attack on the Bonga field, 120 kilometers out to sea.  

Militants began attacking oil facilities in the Niger Delta in late 2005 to demand that more oil revenue be directed to poor local residents.

Shell stopped production at the Bonga field on Thursday.  The field normally produces about 200,000 barrels of oil per day.

Friday, Shell invoked a legal clause that would free itself from contractual obligations to deliver oil from the field in June and July. 

The president of the Association of Nigerian Licensed Customs Agents, Ernest Elochukwu, said the militant attacks deal a "devastating" blow to the country's economy by eroding people's confidence in Nigerian businesses.

The association includes freight carriers, bridge and tunnel operators, and Nigerian corporations. 

 Some information for this report was provided by AFP, AP and Reuters.