BOGOTA, July 5 (Reuters) - Colombia's leftist guerrilla groups have sharply increased their earnings from drug trafficking, extortion and kidnapping in the last four years, the respected news magazine Cambio 16 said on Sunday.

Citing a confidential security service report drawn up last week, Cambio 16 said the rebels' income grew in dollar terms to $790 million last year from $592 million in 1994 when President Ernesto Samper took office, a one-third increase.

In the same period the government nearly tripled is defence budget to $2.65 billion this year from $926 million in 1994, according to Defence Ministry figures.

Colombia's Marxist guerrillas, estimated to total more than 20,000, have increased their power during Samper's government, striking the military with increasing ease and leaving armed forces chiefs to appeal for even bigger budgets, better weaponry and more troops.

Cambio 16 said the war chests of the Revolutionary Armed Forces of Colombia (FARC), Latin America's oldest rebel army, swelled by more than $463 million in 1997 -- at least $312 million of that from drug-running.

The smaller Cuban-inspired National Liberation Army (ELN) had a total income of $341 million, $42 million of that from "war taxes" paid by the oil industry and $98.5 million in extortion payments from the coal-mining sector, Cambio 16 said.

The FARC has denied charges it is a "narco-guerrilla" force with wholesale links to the drug trade.

Marco Leon Calarca, FARC's international spokesman, would not confirm the rebel group's income to Cambio 16 reporters but suggested the military exaggerated the guerrillas' income to try to depict them as common criminals intent on amassing a vast fortune.

Both the FARC and ELN have said they are ready to consider peace talks with president-elect Andres Pastrana once he takes office on Aug. 7 but political analysts say it will take years to finally end Colombia's 30-year-old civil conflict.