GUATEMALA CITY, JUNE 6 -- A military patrol was passing a hamlet in the vast northern department of Peten one recent Sunday morning when it came across a unit of the Rebel Armed Forces, a guerrilla group that has been battling successive Guatemalan governments for 26 years.

In the clash that ensued near the town of Poptun, according to a military communique, four rebels and one soldier were killed, and the patrol recovered an unspecified number of "M16 rifles, knapsacks, munitions and propaganda." Later the same morning of May 24, another patrol clashed with a rebel unit on a road between La Libertad and San Diego in the Peten, leaving six guerrillas and an Army second lieutenant dead, the communique added.

Given what observers here say is the military's history of underreporting its casualties, the sketchy account was taken as an indication of relatively significant fighting.

Despite an Army campaign that has decimated the guerrillas' rural support base, destroyed their infrastructure in the cities, driven tens of thousands of peasants into exile and sharply reduced the rebels' numbers, Guatemalan Marxist-led guerrillas are still operating -- and showing signs of a small resurgence.

A coalition of four rebel groups called the Guatemalan National Revolutionary Union, formed in 1982, has been demonstrating greater cohesiveness lately as it presses on with Central America's oldest insurgency, according to Guatemalan officials.

Late last month, the rebels inaugurated a clandestine radio station, La Voz Popular, that they said would broadcast a weekly program "from the Guatemalan mountains." The initial broadcast May 22 vowed that the rebels would continue their "armed struggle."

The military denies that there has been any significant increase in guerrilla operations, insisting that more frequent clashes lately stem from greater activity by the Army. "What we're doing is taking the initiative," said the defense minister, Gen. Hector Gramajo. "We want to keep the level of the insurgency as low as possible so that the government can exercise power completely."

The Guatemalan Army spokesman, Lt. Col. Roberto Letona, said that the rebels "once were a threat to the government but now they have been turned into a nuisance. It's a victory for them to stay alive."

According to President Vinicio Cerezo, "In some places there was an increase in activities" by the rebels earlier this year, but now it has diminished. "We have the impression that it was to make themselves felt politically, because in Guatemala the process of opening and participation is growing and becoming consolidated at all levels," he said in an interview. "If this goes on, {the rebels} will fall into silence and tranquility and will disappear historically, so they need to make their presence felt."

However, other Guatemalan observers say the continued activities of the guerrillas pose significant problems for Cerezo's 1 1/2-year-old government in achieving national reconciliation. The military's response to the insurgency raises continuing allegations of human rights violations, they say, and the nearly 40,000 refugees camped in neighboring Mexico remain reluctant to return to their homes.

Cerezo, who was inaugurated in January 1986 as the first civilian president of Guatemala in almost 20 years, disclosed that he had "informal contacts" with rebel intermediaries during a visit to Mexico last year, but that nothing substantive resulted.

Cerezo said the intermediaries "raised some ideas on the possibility of opening a discussion about the possible return to the country of some groups linked with the guerrillas." But he said this prospect "was interrupted by a letter that they sent subsequently saying that they were waiting for better conditions" in Guatemala. Cerezo declined to name the intermediaries, but said they were "persons linked to the armed movements."

He said that negotiating with the rebels "in reality is not a crucial point for us, because they are not causing a deep political problem. But we would be willing to sponsor a discussion of terms that would permit their return to national political life.

"We think they have to integrate themselves in the political process of the country. The only things that have to be established are the mechanisms, the guarantees that they need for the process of participation."

Cerezo made it clear that the guerrillas had to lay down their weapons for this process to begin. He said they had countered this by "posing conditions that cannot be accepted."

The guerrillas have repeatedly proposed negotiations with Cerezo's government in the past year and have put forward demands to purge the military and police, eliminate paramilitary and civil defense groups, dismantle the Army's "model villages" and punish those responsible for military abuses. The guerrillas charge that the military opposes any talks and that Cerezo must respect its wishes.

According to Guatemalan sources, the guerrillas operate in at least eight of Guatemala's 22 departments. Under the umbrella group, the Guatemalan National Revolutionary Union, are four distinct guerrilla organizations: the Rebel Armed Forces, the Guerrilla Army of the Poor, the Organization of Armed People and the Guatemalan Workers' Party. The last, the outlawed communist party, is considered an insignificant guerrilla force.

The original guerrilla group, the Rebel Armed Forces, was founded in 1961 by two renegade Army lieutenants. It splintered in the 1970s. Today the Guerrilla Army of the Poor, which operates primarily in the mountainous western part of the country near the border with Mexico, is considered the strongest and most active of the groups. The Organization of Armed People operates mainly in a volcanic zone south of Lake Atitlan, and the Rebel Armed Forces in the vast jungles of the northern Peten.

According to diplomats and military sources, at the peak of the insurgency from about 1979 to 1982, the guerrillas totaled 10,000 to 12,000 fighters and exercised control over some 360,000 people out of Guatemala's population of about 8 million.

In the drives against the guerrillas, it has been estimated that 200,000 people have been killed since the 1960s -- most of them Indian peasants who had been considered part of the guerrillas' rural base.

Following the latest counterinsurgency campaign, the rebels' numbers have been reduced to fewer than 2,000, the sources said. The rebels now are believed to exercise control over fewer than 12,000 people, they added. Gen. Gramajo said, "We cannot say they have been defeated because in Guatemala there is still going to be subversion as long as there is foreign patronage."

According to Cerezo, the rebels have been receiving some outside support, although "these things are difficult to prove." He said that "at the least they have received training and a type of logistical aid from Cuba." Most of the rebels' weapons are standard NATO arms, believed to have been purchased on the international market, military sources said.