By LARRY ROHTER
 
BRASÍLIA, July 23 - After hesitating for six years, in large part because of pressure from the United States, Brazil has announced that it will begin shooting down aircraft used in trafficking illegal drugs in its airspace.

Only Colombia, the source of much of the cocaine and heroin sold in the United States, has such a policy in effect. But Brazil's northern Amazon corridor has become an increasingly busy and essential route in the global drug trade and is used for smuggling arms, gold and diamonds.

The law to permit such an action was originally approved in 1998, but Fernando Henrique Cardoso, who was president from 1995 to 2003, never signed the decree to put the policy into effect. His reluctance was attributed to concern in the United States about the accidental downing of civilian planes, which could expose the American government and companies to lawsuits.

"This is a good measure, and a bold and courageous step by the government," said Gen. Mauro José Miranda Gandra, a former chief of the air force who is now director of the Air Institute at Estácio de Sá University in Rio de Janeiro. But he said he worried that its impact would be "more political than practical" because of restrictions Brazil was imposing on itself.

The Brazilian government's decision to act now seems driven mostly by the deteriorating public security situation in cities like São Paulo and, especially, Rio de Janeiro. Drug gangs there are increasingly powerful and violent, with more firepower than the police, and they have demonstrated an ability to attack police stations and to force businesses and schools to close.

In April 2001, the most notorious of Rio's drug bosses, Fernandinho Beira-Mar, was captured in Colombia in what the authorities described as a guns-for-drugs pipeline involving left-wing guerrillas. Another drug- and gun-smuggling route, said to be one of the most important in supplying Europe with cocaine, runs from Colombia across the northern tier of the Amazon to Suriname.

"Our perception is that we needed to have at our disposal a more powerful means of dissuasion," Brazil's defense minister, José Viegas, said in an interview here. "The drug dealers, knowing that the Brazilian Air Force could not take extreme measures, have felt excessively free at times to come and go over our airspace."

President Luiz Inácio Lula da Silva has referred to the policy as a matter of national sovereignty and security. In an interview with foreign correspondents last year, he expressed annoyance that some pilots of drug-running aircraft were so confident of their immunity from retribution that they would make obscene gestures at the Brazilian Air Force pilots tracking them.

The United States had been cooperating with operations to shoot down drug-running planes in Latin America, but it began to back away from its support in April 2001 when a Peruvian jet shot down a small plane, mistakenly identified as a drug carrier, and killed an American missionary and her child. Because the United States had provided intelligence and technical support to the operation, relatives of the victims sued the United States government and won a settlement.

"U.S. law may forbid assistance to countries that implement shootdown laws under certain conditions," a State Department official said when asked to comment on Brazil's move. The official said, however, that the United States, which is scheduled to provide Brazil with $10.2 million in drug and law enforcement aid this year, "agrees with the Brazilian assessment that the threat posed by drug traffickers is both very serious and increasing," and added, "we have been in consultation with the government of Brazil about the provisions of U.S. law."

Mr. Viegas acknowledged that there had been "difficulties" winning American support for his government's plan, which requires eight precautionary steps before an order to shoot down a plane may be issued. But he said that recent bilateral talks had led to "perfect clarity that the decree will be well received by the American government" and that Brazil would be able to act "without being exposed to commercial sanctions."

Two years ago, Brazil inaugurated the $1.4 billion Sivam radar system, which uses American technology and for the first time allows the government to monitor air activity in the whole of the vast Amazon region. But after an initial decline of 30 percent, which Brazil attributed to traffickers' concerns about the improved tracking capabilities, illegal flights began rising again.

Just last year, Brazil recorded 4,128 "unauthorized flights," some of which were innocent violations like those made by ranchers in the Amazon flying from one plantation to another. Mr. Viegas said, however, that there had been a "real increase" of deliberate violations of Brazilian airspace, apparently by drug smugglers who realized that the government's hands were tied.

The new policy will go into effect in late October, and will be preceded by a publicity campaign to warn ranchers and others of the need to file flight plans. Brazilian officials have also expressed hope that during the interim period the United States will formally endorse the new policy here.

Brazilian officials made it clear that the new policy would not be applied against any aircraft with children on board. Though Mr. Viegas said that was "a necessary limitation" and "an incidental question," some other prominent supporters of the law were skeptical.

"This really left me perplexed, because it practically undermines the very purpose of the decree," General Gandra said. "What you're doing is creating a safe-conduct pass for drug-smuggling aircraft carrying kids and creating the possibility that children will be kidnapped and used as human shields."

Even with the caveat for children, the constitutionality of the law is being questioned. Brazil forbids the death penalty as punishment for criminal acts, and though the government argues otherwise, some legal experts and other commentators maintain that the statute amounts to a de facto execution of drug traffickers.

"The ethical and juridical problems raised by the regulation of the shoot down law are much greater than the benefits this extreme measure can bring," the daily O Estado de São Paulo said in an editorial this week. "This penalty will be applied beyond the reach of justice, by administrative decision of the commander of the air force, who will have life and death power over crew members and passengers of irregular flights."