KUALA LUMPUR (AFP) - Malaysia's highest court on Thursday upheld the conviction and death penalty of three Mexican brothers, a Singaporean and a Malaysian in a high-profile drugs case.

The Gonzalez Villarreal brothers - Luis Alfonso, 47, Simon, 40, and Jose Regino, 37 - were arrested in March 2008 at a factory in southern Malaysia where police found 30kg of methamphetamine and equipment to make drugs.

The siblings insist that they were merely working as a cleanup crew and that they were unaware drugs were being made in the factory.

Drug trafficking carries a mandatory sentence of death by hanging upon conviction in Malaysia. They were sentenced in May 2012 and the ruling was affirmed a year later.

"Our decision is unanimous. Appeal dismissed against all five defendants," Federal Court judge Zulkefli Ahmad Makinudin told the court in Malaysia's administrative capital of Putrajaya.

The Mexican foreign ministry has said it had "repeatedly expressed (to Malaysian authorities) Mexico's position against capital punishment".

The brothers are from Culiacan, capital of the northwestern state of Sinaloa, which is known as the bastion of the powerful Sinaloa drug cartel.

The defendants included Singaporean citizen Lim Hung Wang, 56, and Lee Boon Siah, 51, of Malaysia.

Lawyers for the Mexican brothers said they would seek a judicial review of the case by the Federal Court.

Failing that, they may consider seeking a royal pardon from Malaysia's figurehead monarchy.

Success in both cases is extremely rare.