BOGOTÁ, Colombia, March 27 — Two American pilots who flew the same United States government planes that crashed in rebel-held territory in the last six weeks warned late last year that the aircraft should have been replaced, American officials confirmed today.

The pilots had called for replacing the single-engine Cessna 208's that are used on intelligence missions in Colombia by their employer, California Microwave Systems, a subsidiary of Northrop Grumman. Northrop is under Pentagon contract to operate reconnaissance flights over Colombia's vast coca fields as part of Washington's war on drugs.

In letters sent in November and December, the pilots had called for an "increase in the safety margin" and suggested the use of the two-engine Beech King Air 300, an American official familiar with the letters said. That plane can clear Colombia's high mountains and operate on one engine.

The use of Cessna 208's on drug surveillance missions over Colombia's southern jungles continued, however, and on Feb. 13 one of the planes went down after its engine failed. Leftist rebels operating in the area killed two crew members, one of them an American, and took three other Americans hostage.

Then on Wednesday, another Cessna 208 on a search-and-rescue mission for the missing Americans apparently suffered engine trouble and crashed. Three American crew members, all of them described as private contractors, were killed.

The troubles with the planes, first reported today in The Los Angeles Times, have prompted two members of the United States Congress to propose legislation that would limit United States involvement in Colombia.

Representative Janice D. Schakowsky, an Illinois Democrat, will reintroduce a bill barring the government from using private contractors in Colombia. Meanwhile, Jim McGovern, a Massachusetts Democrat, called on Congress to remove military financing from the supplemental appropriations bill submitted by the Bush administration this week.

The Bush administration has asked for $104 million in additional funds to help Colombia in its war against drugs and terrorism, a modest overall increase in American support to Colombia. For fiscal 2004, President Bush has requested over $700 million for Colombia, not including this latest supplemental.

Much of the more than $2 billion the United States spent in Colombia since 1997 has gone to private companies under contract with the United States government.

American and other foreign workers employed by those firms do everything from handling administrative duties to running rural development programs to flying aircraft on antidrug missions, which has taken the lives of three Americans.

In early March, the warnings from the California Microwave pilots reached officials at the Southern Command in Miami, the United States military's base of Latin America operations.

A spokesman for the Southern Command, Raul Duany, said the command had Northrop Grumman halt use of the Cessna 208's as mechanical checks were made. Soon after, the use of the plane was again approved.