By Walter Bianchi

BUENOS AIRES (Reuters) - Argentina complained on Friday about military exercises that Britain is planning this month in the disputed Falkland Islands in the South Atlantic.

Argentina has for decades claimed sovereignty over the British-run islands it calls the Malvinas and the dispute led to a brief war in 1982. The overwhelming majority of the islands' 3,000 inhabitants say they want the islands to remain a British overseas territory.

Argentina's foreign ministry said in a statement that it had sent a letter to the British ambassador demanding the country call off the "illegitimate" exercises, which are scheduled for Oct. 19-28 and include the launching of Rapier missiles. A spokeswoman for the British embassy in Buenos Aires called it a "routine exercise" that takes places about twice a year.

Argentina lodged its complaint just a month after the two countries agreed to work together toward removing measures restricting the oil and gas, shipping and fishing industries on the remote islands.

Pro-business President Mauricio Macri has sought to improve relations since taking over in December after diplomatic tensions mounted under his predecessor, populist Cristina Fernandez.

But in a blunder that embarrassed the administration last month, Macri claimed he and British Prime Minister Theresa May had agreed, during a brief encounter at the United Nations General Assembly, to discuss the sovereignty claim.

Argentina's Foreign Minister Susana Malcorra later walked back the remarks after Britain denied that the issue of sovereignty had come up.

In Friday's statement, the foreign ministry said it had learned of the planned military exercises on Thursday. It said the exercises "contradict the principle of peaceful conflict resolution" and called the Falklands "Argentine territory illegitimately occupied by the United Kingdom."

In last month's agreement, the two sides also agreed to establish additional flights between the Falklands, located about 435 miles off the coast of southern Argentina, and third countries in South America.

The last bout of serious tension over the Falklands occurred in June last year when an Argentine federal judge ordered the seizure of millions of dollars in assets owned by oil drillers operating in the area.

(Additional reporting by Maximiliano Rizzi and Luc Cohen; editing by Grant McCool)