Despite Colombian and U.S. efforts to stem the flow of drugs and guns that have helped build Latin America's most powerful guerrilla insurgency, the Marxist-led rebels are amassing a sophisticated arsenal from new sources in the the former Soviet Bloc that is rapidly changing the balance of power in Colombia's civil war, officials say.

The rebels' new firepower was first put on public display last fall, when guerrillas from the Revolutionary Armed Forces of Colombia (FARC) emerged from the jungle for the opening of peace talks. Colombian and U.S. officials were stunned to see parading rebels carrying thousands of new AK-47 assault rifles, Dragunov sniper rifles and other weapons from Eastern Europe and the former Soviet Union.

"We thought the parade was a special group sent out to impress us, and it did," said a U.S. official. "But it has become clear they have access to a whole new supply of weapons, and it is a serious escalation. They are far past what the army has."

Colombian, U.S. and Russian intelligence officials say that new weapons from Eastern Europe are surging into Colombia. Largely out of fear that the FARC is gaining the upper hand in the conflict, the United States is rapidly escalating aid to the Colombian army. In addition to $289 million in aid already committed for fiscal 1999, the Clinton administration is seeking a $2 billion package over the next three years.

The rebels' arms purchases have increased in part because of growing cocaine consumption in Europe, especially in former Soviet Bloc countries, coupled with the widespread availability of weapons for sale across Eastern Europe.

"What we are seeing is a heavy push of cocaine into Europe and at the same time we are seeing Russian criminal groups seeking new alliances with Colombian cartels," said Gen. Rosso Jose Serrano, Colombia's national police chief. "We don't know how much is guns for drugs and how much is just taking their vast wealth to the black market and buying weapons, but it is a serious situation for us."

The result, according to U.S. and Colombian officials, is that the FARC and paramilitary groups organized to combat the guerrillas have a secure arms pipeline. The officials say the pipeline delivers vast quantities not only of assault rifles but also of heavy machine guns, hundreds of thousands of rounds of ammunition, small artillery, explosives, hand grenades and rocket-propelled grenades, significantly altering the balance of power against the government in Colombia's civil war.

Colombian and U.S. officials also cited consistent intelligence reports that the FARC possesses SA-14 shoulder-launched surface-to-air missiles, although none has been captured. The use of such missiles, said one U.S. official, "would change the threat envelope considerably" for the U.S.-backed military and the estimated 200 U.S. trainers who routinely rotate through Colombia and use helicopters to move around the country.

In addition to the Eastern European weapons, the FARC has significantly upgraded its communications equipment, buying Japanese and European encryption technology, voice scramblers and other technology that makes interception of their communications almost impossible, Colombian and U.S. intelligence analysts said. The group also has a fleet of single-engine airplanes and several helicopters.

In recent months the Colombian anti-drug police found a shipment of 480 new Ukrainian-made AK-47s near the Caribbean port of Turbo, an area controlled by the right-wing paramilitary groups.

For most of its 35 years, the FARC was a ragtag, rural Marxist band relying on outdated weapons bought on the black market and from disbanded Central American insurgencies. But today the FARC is deeply involved in the drug trade in Colombia, which produces 80 percent of the world's cocaine, and is able to spend the profits on supplying its 15,000 fighters. The FARC makes millions of dollars a year from taxing the production of cocaine, as well as charging to protect clandestine laboratories, airstrips and drug-trafficking routes.

Peace talks begun last year between the government of President Andres Pastrana and the FARC were suspended in July. However, the talks were reinitiated on Oct. 24 when the two sides were to discuss a 12-point agenda on how to end the war, which has claimed the lives of tens of thousands of people, mostly civilians. The paramilitary groups, about half the size of the FARC, are more directly involved in drug trafficking, according to the U.S. Drug Enforcement Administration (DEA). They operate cocaine laboratories and are partners of important international drug-trafficking organizations, according to DEA intelligence reports.

Barry R. McCaffrey, the Clinton administration's drug policy coordinator, said Europe now consumes between 80 and 130 tons of cocaine a year, a sharp increase in the past year. Colombian intelligence officials estimate that at least 10 tons are shipped through Russia.

Some of the cocaine stays there to meet growing Russian demand. But most is shipped into Europe from the east. Russian criminal groups reap enormous profits either way by controlling the routes.

Drug traffickers pay cash or exchange cocaine for weapons and bring the guns back in the same transport containers they use to bring the drugs over.

"The Russian military has been involved in gray market sales of weapons since the fall of the Soviet Union," said Rens Lee, president of Global Advisory Services and an expert on Russian arms sales. "They want to make money and are not fastidious about whom they sell weapons to. Military profiteering is very important."

The drug traffickers either auction off weapons to the highest bidder or turn them over to a group that has paid in advance, intelligence officials said.

Dimitri Belov, a spokesman for the Russian Embassy here, said Russia is working closely with Colombian police.

"We are not satisfied with the current situation but in some ways it is exaggerated, too," Belov said. "We feel the situation between Russia and Colombia is under control."

Colombian intelligence reports show that most of the guerrillas' weapons are delivered from Ecuador by river or small airdrops into the southern Putumayo and Caqueta regions. Several large airdrops of several hundred weapons each have been detected in the past three months, intelligence officials said. The paramilitary groups are largely supplied by ships that unload their cargo in small, uncontrolled ports in northern Colombia, mostly in the Cordoba region.

Colombian intelligence reports say the arms network is an extension of contacts between Colombian and Russian organized crime groups.

CAPTION: In a rare photograph, FARC military leader Jorge Briceno Suarez, fourth from right, stands with senior commanders.