The Clinton administration is preparing to greatly step up military and economic aid to Colombia in response to fears that the growing strength of drug-financed Marxist guerrillas there could undercut counternarcotics efforts across the Andean region.

In separate visits to Colombia, senior U.S. officials warned President Andres Pastrana last week that he risks losing U.S. support if he makes further concessions to the insurgents in an effort to restart stalled peace negotiations, according to sources familiar with the talks. But the officials, White House drug policy director Barry R. McCaffrey and Undersecretary of State Thomas Pickering, also told Pastrana the United States will sharply increase aid if he develops a comprehensive plan to strengthen the military, halt the nation's economic free fall and fight drug trafficking.

Part of the economic aid will be $3 billion in International Monetary Fund loans, with some additional direct U.S. military aid. Pickering, briefing reporters here, said he asked Pastrana to present his plan by the middle of September in order to seek supplemental funding from Congress this year.

While specific aid figures will not be discussed until Pastrana presents his plan, senior officials and congressional sources said it would be hundreds of millions of dollars. Colombian defense officials last month requested $500 million in additional military aid over the next two years, a number U.S. officials said is being discussed. U.S. security assistance already stands at $289 million this year, making Colombia the third-largest recipient of such U.S. aid after Israel and Egypt.

"We are working toward a much larger engagement with the United States, involving combating narcotics, strengthening our battlefield capabilities and economic issues," said a senior Colombian Foreign Ministry official. "It is a much broader engagement than just the narcotics issue because all our problems are linked."

President Clinton, for the first time that senior administration officials could recall, was briefed on Colombia by cabinet officials Wednesday. The officials said national security adviser Samuel R. "Sandy" Berger and Pickering, the State Department's third-ranking official, were put in charge of interagency coordination of Colombia policy.

The decision to escalate aid comes a year after the United States resumed helping the army and expanded intelligence sharing, ending a period covering most of this decade during which collaboration was cut off because of the army's human rights record.

Currently the United States is training a 950-man Colombian army counternarcotics battalion, the first such specialized unit in the military, whose primary objective will be to regain control of guerrilla-controlled territory. Pentagon and State Department officials said they recently agreed to try to provide the group with 18 Huey UH-1N helicopters. And, according to the same sources, the United States is planning to fund at least two more such battalions, a move that would boost U.S. aid by tens of millions of dollars.

Pickering said he was "sobered but certainly not panicked" by his trip and stressed that the guerrillas are not on the verge of military victory. But other officials were less optimistic.

"Colombia is a disaster, and I don't see any way around that," said McCaffrey, a retired general who recently proposed spending an additional $1 billion in the Andean drug-producing region, with about half of the money going to Colombia. "We are in a period of intense debate in the administration and on the Hill . . . but we don't have the latitude to let a fellow democracy go under."

McCaffrey, in an interview, said his proposal, including money for alternative development and judicial reform along with military aid, was an attempt to tackle Colombia's multiple problems.

"So far the debate has been at a micro level, about 10 helicopters here or training a battalion there," McCaffrey said. "We are not talking about the right order of magnitude for this problem."

Colombia produces 80 percent of the world's cocaine and about 70 percent of the heroin sent to the United States. Two Marxist guerrilla groups -- the Revolutionary Armed Forces of Colombia (FARC), with about 15,000 combatants, and the National Liberation Army (ELN), with about 5,000 -- control about 40 percent of the national territory and receive hundreds of millions of dollars a year from protecting drug trafficking routes, airstrips and laboratories. In addition, 7,000 right-wing paramilitary troops, who also derive millions of dollars from cocaine trafficking, control about 15 percent of the national territory.

With the U.S. military gone from Panama, officials say, the FARC is increasing its use of the southern portion of that nation as a safe haven, while expanding its presence in neighboring Ecuador and Venezuela. And Venezuelan President Hugo Chavez, a populist leftist who has expressed sympathy for armed revolution, has denied use of Venezuelan airspace to U.S. planes pursuing drug traffickers.

"We have a disaster in our back yard, and it is an incredibly dangerous situation," said Rep. John L. Mica (R-Fla.) in an interview. "It is compounded by the mess in Panama, with Howard Air Force Base being shut down, and Chavez in Venezuela is thumbing his nose at us. The question is, will we move too late to avoid an even bigger disaster?"

But human rights organizations say tilting aid heavily toward the military is dangerous.

"In a world with a lot of bad policy options toward Colombia, the United States is taking the worst one," said Winifred Tate of the Washington Office on Latin America. "By strengthening the military you are strengthening an abusive, corrupt institution that has resisted civil control and human rights reforms. This is not going to achieve U.S. policy objectives."

While policy is being debated, officials in the Pentagon, State Department, White House and Congress said consensus already has emerged on some points.

One is that Pastrana gave away too much to the FARC in an effort to negotiate peace, including allowing the guerrillas to occupy a demilitarized zone the size of Switzerland. The concessions so angered the military that in May Defense Minister Rodrigo Lloreda resigned in protest, sparking an unsuccessful coup plot by senior military officers.

There is also agreement that, while U.S. aid should be focused on fighting drugs, the line between counternarcotics and counterinsurgency has blurred so much that it is almost meaningless because of the FARC's deep involvement in the drug trade.

In the past two years the FARC has taken over the vast jungle regions of Putumayo and Caqueta in southern Colombia, and their presence has brought a huge increase in the amount of coca -- the raw material for cocaine -- under production there. Because of the FARC's strength, the government has been unable to maintain a presence in the region.

That is due, in part, to the weakness of the Colombian military.

"Intelligence assessments have identified numerous deficiencies in training, force structure, leadership, intelligence, mobility and communications which must be corrected if the government of Colombia is to mount credible counter drug operations," McCaffrey wrote in a July 13 letter to Secretary of State Madeleine K. Albright. "Unless the government of Colombia succeeds in establishing a security presence in the coca growing regions, Colombian coca cultivation will continue to expand and the guerrilla movement will continue to strengthen."

To counter that, the heart of any immediate increase in military aid will focus on upgrading a sophisticated intelligence and listening post in Tres Esquinas, on the edge of Putumayo province, and Pentagon training of new, special units in the Colombian army, administration officials said.

CAPTION: DANGEROUS EXPORT (This graphic was not available)