Millions of mobile phone users in Buenos Aires found themselves without service Tuesday, because of an unexplained outage to wireless provider Claro.

Claro, part of Mexican-owned America Movil, boasts of being the leader in Argentina's 20-million-user mobile phone market, but beginning as early as Monday night many of its users in Buenos Aires and its suburbs experienced a prolonged break in voice and data service.

"Checks verified interruptions in service that affected an important number of Claro clients. So it was ordered to restore service immediately," the National Communications Commission said.

The company issued a brief statement Tuesday saying the problem had been resolved and service would be gradually restored.

It was not immediately clear what caused the breakdown.