(Adds details of plan to cut output)

SAO PAULO, May 18 (Reuters) - Brazil's Usinas Siderurgicas de Minas Gerais S.A. will turn off two of its furnaces to reduce pig iron production by 120,000 tonnes per month and adapt the operation to lower demand in the steel market, the company said on Monday.

Usiminas, as the company is known, said it will turn off the number 1 furnace at its Cubatao plant in Sao Paulo state and the number 1 furnace at Ipatinga plant, in the state of Minas Gerais, both located in Brazil's center-south region.

"Such adjustment aims to adapt production to the current rhythm of demand in the steel market," the company said in a filling with the local market regulator in Brazil.

Usiminas produces some 6 million tonnes of raw steel a year and sells around 80 percent of that in the local market.

The company, which also produces iron ore, posted a net loss of 235 million reais ($78.1 million) in the first quarter, due to falling domestic steel demand and weaker iron ore prices.

(Reporting by Marcelo Teixeira; Editing by Ken Wills)