By Pratima Desai

LONDON (Reuters) -Glencore declared force majeure on some deliveries of cobalt from Democratic Republic of Congo days after the government suspended exports of the battery material, three sources familiar with the matter told Reuters.

Congo, the world's largest cobalt producing country, introduced a four-month ban on all cobalt exports in February in an attempt to curb a supply glut that helped send prices to nine-year lows and stifled its tax revenues.

As a result of the ban, London-listed Glencore took the rare step of declaring force majeure on some supply agreements for cobalt produced at its Congolese operations, invoking a measure meant for unforeseeable circumstances that prevent a contract's execution, the sources said.

Glencore declined a Reuters request for comment.

Glencore, the world's second-largest cobalt producing company, mined 35,100 metric tons of cobalt contained in concentrate and hydroxide at its Congo operations last year.

Many of its customers are still receiving cobalt under their contracts, the sources said. And while others are not, it was unclear to what extent the supply issue had affected their operations.

Cobalt is a byproduct of copper production in Congo, which accounted for 220,000 tons, or 78%, of global cobalt output last year.

In metal form, it is used to manufacture parts for aerospace and military equipment. Most of the cobalt produced in Congo, however, comes in the form of hydroxide and is used to make chemicals for batteries used in electric vehicles and mobile devices.

Growing surpluses - partly due to lower than expected demand for electric vehicles and a supply surge from operations owned by China's CMOC Group - drove down cobalt prices to nearly $10 a pound or $22,000 a ton in February.

Congo's export ban and a force majeure declaration in March by Eurasian Resources Group have since helped prices recover by around 35% to trade at $15.8 cents or a pound or $34,832 a ton on Wednesday.

Congo has not said whether the export suspension will be extended when the ban ends on June 22, or if the government would look at export quotas.

(Reporting by Pratima Desai; Editing by Veronica Brown and Joe Bavier)