Bolivia has narrowed to six from eight the list of competing foreign firms vying to tap its vast lithium resources, Energy Minister Franklin Molina said on Tuesday, part of the country's most ambitious effort yet to extract the metal.

Bolivia has the world's largest resources of lithium, the ultra-light metal key to making batteries for electric vehicles, but has struggled for decades to mine them commercially.

Under left-wing President Luis Arce, Bolivia has sought to partner with foreign firms to jumpstart its mining capacity. Last year, he announced a shortlist of eight companies from the United States, China, Russia and Argentina that, if chosen, would extract the metal using an untested technology known as Direct Lithium Extraction.

In a press conference, Molina did not specify which two companies had been disqualified. He said final results are expected to be announced on June 15, itself a delay on an initial May deadline for the announcement.

Bolivia still faces an uphill battle to exploit its lithium, including legal obstacles and a patchy track record for multinational firms operating in the Andean nation.

Under Bolivian law, only state-owed lithium firm Yacimientos de Litio Bolivianos (YLB) is allowed to extract the metal. The government has yet to announce how they would square that restriction with partnering with foreign firms.

Bolivia has also fallen behind neighbors Argentina and Chile, which have built substantial lithium operations at a time when prices are skyrocketing. Chile is the world's No. 2 lithium producer and Argentina has a promising pipeline of projects already in the worlds.

The initial list of finalists in Bolivia includes US startups Lilac Solutions - backed by German carmaker BMW and Bill Gates' Breakthrough Energy Ventures - and EnergyX, as well as China's giant battery maker CATL.

The other companies are Argentina's Tecpetrol, Russia's Uranium One and Chinese ventures Fusion Enertech, TBEA and CITIC Guoan Group.