UPDATE 2-Bolivia names new head of state energy firm

(Adds quotes, details, background)

By Eduardo Garcia

LA PAZ, March 23 (Reuters) - Bolivian President Evo Morales sacked the head of the state energy firm on Friday, naming a fourth person to the post since the government nationalized the country's oil and gas fields less than a year ago.

The switch came a week after the outgoing head of YPFB, Yacimientos Petrofileros Fiscales Bolivianos, took the blame for costly errors in complex contracts Bolivia signed with the foreign companies that exploit its natural gas fields.

"There are mistakes, we admit that there are mistakes, but I want to say to the Bolivian people... there will never be betrayals. The nationalization will carry on," Morales said at a news conference.

Morales took office 14 months ago after winning elections on promises to take control of the country's rich natural resources and spread energy income to the indigenous majority in South America's poorest country.

The nationalization took place in May but has been hampered by a series of setbacks and delays that experts have blamed on the lack of technical expertise at YPFB and the shortage of funds.

The latest delay is that the opposition-controlled Senate is balking at passing an amendment that will fix faults in contracts signed with foreign operators such as Brazil's Petrobras (PETR4.SA) (PBR.N), France's Total (TOTF.PA), and Spain's Repsol YPF (REP.MC).

Under the contracts, signed late last year, the companies will pay a much higher share of their profit to the Bolivian state and their operations will be controlled by the government.