The U.S. Navy is taking a more aggressive approach in its anti-piracy operations in the Gulf of Aden, and plans are being drawn up to put suspected pirates on trial, a senior navy commander said on Thursday.

The U.S State Department in January signed an agreement with Kenya on putting suspected pirates on trial there, allowing the U.S. Navy to begin taking them into custody on the high seas.

"We're really looking forward to the capture and prosecute mechanism to drive the number of attempts down," Vice-Admiral William Gortney, commander of the U.S. Navy's Fifth Fleet, told a news briefing in Bahrain.

"The games have changed for the pirates," Gortney said.

A naval force consisting of U.S. and British ships last week for the first time seized two groups of suspected pirates after receiving distress calls from merchant vessels in the Gulf of Aden.

Alarmed by the audacious capture of a supertanker last year, foreign navies patrolling the busy shipping lanes off the coast of Somalia have been taking a more robust approach to piracy for several months.

British forces handed over a group of pirates to Kenya in December and the French navy took gunmen it had captured to the semi-autonomous northern Somali region of Puntland in January.

Gortney said successful piracy attempts had become significantly fewer this year as the shipping industry is increasingly using self-protection measures, such as speeding manoeuvres and barbed wire.

There have been 21 piracy attempts this year, only three of which were successful, according to the U.S. Navy. This compares with 42 successful attempts out of a total of 120 attempts last year.

The suspected pirates captured last week were being held in a detention facility on board a U.S. vessel.

Gortney said the U.S. Navy is confident it has enough evidence to try a group of seven suspected pirates that last week attacked the Marshall Islands-flagged vessel Polaris in the Gulf of Aden.

"The strongest air-tight case is when we catch them in the act and we have eyewitnesses that are willing to testify against the pirates, we think we have that in (this) case", he said.

It was still unclear whether there was enough evidence to put a second group of nine suspected pirates on trial, he said.

"We found the alleged pirates had machineguns and rocket-propelled grenades and ladders and (we think) all that is not very good ... stuff to fish with, but those may not (meet) the rules of evidence that Kenya would like," he said.