"These pirates are terrorists and there is no need to negotiate with them. Attacking them will solve future piracy plans," Yusuf said.
The yacht, Le Ponant, includes 30 crew members and lies anchored off Puntland, which has proclaimed autonomy from the rest of the country.
Somalia, which lies at the mouth of the Red Sea on a major trade route between Asia and Europe via the Suez Canal, has not had a functioning government since the 1991 ousting of Mohamed Siad Barre, the last president.
Civilians killed
The pirates attempted to come ashore on Sunday, but residents said fighters working for the local authorities made it clear they were not welcome.
"The pirates opened fire, killing two men after local fighters told them to go away," Mohamed Ibrahim, a radio operator, said.
Ibrahim said the men onshore did not return fire.
French counter-terrorism and hostage-rescue unit troops have been sent to Djibouti and are awaiting further orders.
Bernard Kouchner, the French foreign minister, has not ruled out the payment of a ransom to secure the release of the crew, which include 22 French nationals and around 10 Ukrainians.
Herve Morin, the French defence minister, said that there could be no military intervention unless the safety of the crew could be guaranteed.
France said it had made radio contact with the ship on Sunday and that a frigate, Le Commandant Bouan, and a military aircraft had been sent to trail Le Ponant.
Security plea
In a related development, the International Maritime Bureau (IMB) has called for an increase in security on the northern Somali coast, saying it has become increasingly dangerous.
The IMB's Malaysia-based Piracy Reporting Centre said it was possible the culprits were the same people responsible for a spate of recent attacks off the northern Somali coast.
Noel Choong, the centre's manager, said anti-piracy activities, including patrols by Western warships, had suppressed incidents on Somalia's east coast, but pirates had now headed north to the Gulf of Aden.
"Definitely this year we have seen a shift from the east to the north ... and that's a lot more dangerous because it's a main shipping route," Choong said.
"There are a lot of tankers carrying vulnerable cargo, and we are worried about an environmental disaster if there's any attack on chemical or crude oil tankers.
"The UN Security Council or someone must take some form of action to stop these pirates from attacking innocent seafarers."