Berlin - German prosecutors applied Tuesday for arrest warrants for seven Somali pirates who attack a German naval vessel in the Gulf of Aden last month.

The group is on board a German frigate due to tie up in the Kenyan port of Mombasa on Wednesday.

Justice officials said the move was necessary, otherwise the suspects would have to have been freed when the frigate Rheinland-Pfalz docks.

A German air force plane and 40 policemen were standing by to fly to Kenya to pick up the pirates if the Kenyan government refuses to accept them and put them on trial.

Under a deal with the European Union, Kenya has pledged to try pirates detained by EU warships for attacks on merchant shipping off the Somalia coast.

It is unclear whether the arrangement applies to attacks on warships. The pirates currently on the German frigate were detained over a week ago after an unsuccessful attack on the navy oil tanker Spessart.

On March 9, Germany handed over to Kenya nine pirates captured off Somalia after another botched attack, this time on a German freighter.

Piracy has started to pick up again off Somalia in recent weeks following a lull of several months.

Pirates in 2008 seized dozens of ships and earned tens of millions of dollars in ransom, prompting the international community to send in warships to the region.