MOMBASA, march 11 (Reuters) -- A Kenyan court sentenced seven Somalis to 20 years in prison for piracy on Wednesday after they tried to attack a Danish cargo vessel.

British Royal Navy forces arrested the men in 2008 after they attempted to seize MV Powerful off the Gulf of Aden, one of the world's busiest shipping routes. Two pirates died in an ensuing fight.

They were then handed over to Kenyan authorities and charged with piracy.

"Having considered the seriousness ... of the offence, and circumstances under which the suspects were arrested, only stiff penalties can deter such activities," Senior Principal Magistrate Lilian Mutende said, delivering her judgment.

Pirates have caused havoc in the Gulf of Aden, raking in millions of dollars in ransoms, hiking insurance premiums on shipping and threatening humanitarian supplies.

Kenya is holding over 100 suspected pirates, and police say this is clogging jails and courts. Local Muslim leaders say Kenya should not be used as a dumping ground and foreign navies should take charge of the people they arrest.

International navies trying to counter piracy off Somalia are often reluctant to take suspects to their own countries because they either lack the jurisdiction to put them on trial there, or they fear the pirates may seek asylum.

The European Union, United States and some other countries have instead struck agreements with Kenya to hand over suspects to face trial there. Some pirates are being prosecuted in France and the Netherlands.

In Kenya, 10 other pirates are serving a seven-year jail term at a prison in Voi, near Mombasa.

A lawyer representing the seven, in their 20s and 30s, said he planned to appeal against the sentences.

"It is clearly stated in law that the court in Kenya ... has no jurisdiction beyond the Kenyan waters. Why should Kenya be the one to feed Somali aliens for 20 years?" Jared Magolo said.