Two Somali men convicted of attempting to hijack an oil tanker operated by a Japanese company in the Arabian Sea in 2011 have appealed the 10-year prison term they were given this month, court officials said Friday.

Mohamed Urgus Adeysey and Abdinur Hussein Ali, who claim to be Somali nationals, filed the appeal Thursday with the Tokyo High Court. When they were captured aboard the tanker by elements of the U.S. Navy, the two, plus two cohorts, were not carrying documentation that could establish their nationality. The other two, including one who may have been a minor at the time they boarded the ship, are being tried separately in Tokyo.

Earlier this month, the Tokyo District Court, in a lay judge trial, sentenced Adeysey and Ali to 10 years, ruling their attempt, in concert with the two others captured, to seize the tanker and hold its crew hostage in order to gain a ransom was a common act of piracy off Somalia.

The three professional and six lay judges ruled the pair's actions were "heinous."

The four boarded the Bahamas-registered tanker Guanabara, operated by Mitsui O.S.K. Lines Ltd., on March 3, 2011, firing automatic rifles in a bid to intimidate its 24-member crew, who locked themselves in a safe cabin aboard ship.

The four were taken into custody the following day by the U.S. Navy, which was responding to an SOS signal. They were subsequently handed over to Japanese authorities.

Under the nation's antipiracy law, which took effect in 2009, acts of piracy that involve the commandeering of a vessel are punishable by five years to life in prison.