Brazil's government says its crackdown on illegal logging and tighter law enforcement have this year slowed deforestation of Brazil's Amazon by half after it reached its second-highest level ever the year before.

"We've cut the backbones of these gangs," Environment Minister Marina Silva, flanked by federal police chiefs, told a press conference.

Environmental groups say much of the reduction in Amazon deforestation is due to a slump in farming rather than government action. They fear Brazil's agriculture frontier will speed its advance on the rain forest once commodity prices improve.

Wednesday's large-scale police operation was the fifth launched against illegal Amazon logging since President Luiz Inacio Lula da Silva entered office in 2003 with strong backing from environmentalists.

As a result of the crackdown, the center-left government estimates up to 60 percent of Amazon timber is now produced legally compared with around 20 percent in 2000. It expects to quadruple the amount of illegal lumber confiscated this year compared with 2002, the final year of the former government.

Lula's support for Amazon road and energy projects has worried some environmentalists that his government may ultimately increase access to the jungle and speed its destruction.

His environment minister advocates controlled use of the rain forest rather than banning logging outright. Silva is also pushing for a new law to better control public lands, which make up 75 percent of the Amazon area.

A recent study by the Carnegie Institution of Washington suggests damage to the rain forest may be twice as large as previously thought due to undetected selective logging, where individual trees are picked out of the forest.

The government contests research suggesting selective logging drastically increases rain-forest damage and says it is preferable to clear cutting where all trees are felled.