A Spanish naval frigate is heading for the area off Somalia where pirates seized a Spanish fishing boat with 26 crew on board, officials say.
The captain of the trawler - contacted by Spanish radio - said the crew were "all well". The Somali pirates have demanded a ransom.
The Playa de Bakio - a Basque tuna boat - was attacked about 250 miles (400km) off the coast on Saturday.
In a separate attack, a Japanese oil tanker was damaged by a rocket.
None of the tanker crew were reported injured in the night-time attack - blamed on Somali pirates - in the Gulf of Aden on Monday.
Somali coastal waters are among the most hazardous in the world.
The Spanish trawler's crew consists of 13 Spaniards and 13 Africans.
The Spanish government is now trying to locate the boat.
A Basque regional government source said the trawler was in "international waters" when it was attacked.
Four armed pirates seized the boat using grenade launchers, the source from the Basque government's agriculture, fisheries and food department told the Spanish news agency Efe.
The trawler suffered "some damage" in the attack but is "navigable", the source said.
It comes a few days after a Paris court charged six Somalis with taking a French luxury yacht's crew hostage earlier this month.
The yacht's 30-member crew were held hostage for a week but released after its owners apparently paid a ransom of $2m (£1m; 1.3m euros).
The pirates were then picked up by French commandos in a helicopter raid.
Last year, more than 25 ships were seized by pirates in Somali coastal waters.
Somalia has not had an effective central government for more than 17 years and is plagued by insecurity.