(Recasts with U.S. Navy statement on captured suspects)

SEOUL, May 14 (Reuters) - Seventeen suspected pirates were captured in the Gulf of Aden after South Korean and U.S. Navy vessels helped an Egyptian ship under attack, the navies said on Thursday.

The U.S. Fifth Fleet said in a statement a boarding team of missile cruiser USS Gettysburg found rifles and a rocket-propelled grenade on a ship suspected to have served as the mother ship for a failed attack on Egyptian-flagged vessel Amira.

The Amira had reported on Wednesday that it had been under attack by pirates who fired with rifles and grenades at the ship and tried to board it by throwing a rope to its deck.

The U.S. Navy said all suspected pirates were brought on board the Gettysburg for interrogation.

The difficulty of providing evidence of piracy acts that hold up in courts has been one of the hurdles faced by international navies that rushed to the region after a surge in piracy attacks in the Gulf of Aden and off the coast of Somalia.

Pirates have typically used ships large enough to navigate the high seas to launch smaller skiffs to board merchant vessels.

The South Korean destroyer began patrolling around the Gulf of Aden in April in what military officials said was the country's first naval mission abroad.

In May, the destroyer came to the rescue of a ship from political rival North Korea by chasing away pirates that had targeted one of its cargo vessels, the South's military said.

The Gulf of Aden is a key shipping route for South Korean vessels as they sail from the Middle East with crude oil.

About 460 South Korean vessels pass the gulf every year, according to government data.

South Korean cargo vessels have been captured by Somali pirates in recent years and sailors held hostage for ransom. In February, South Korean sailors were among 23 who were released after being held for months. (Reporting by Jon Herskovitz and Kim Junghyun; Editing by David Fox)

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