Antigovernment rebels have kept alive their war in northern Nicaragua and in some ways have fought even more effectively during the six months since the U.S. Congress cut off their CIA funds, according to government officials, diplomats and other military observers.

Thousands of guerrillas of the Nicaraguan Democratic Force have maintained a steady presence in sparsely populated mountains near Honduras and at some points as much as 60 miles inside Nicaragua, and they recently have stepped up cross-border infiltration in preparation for their third offensive since the funding cutoff, Nicaraguan Army and security officials said.

The force -- commonly known by its Spanish initials, FDN -- apparently has obtained funds from private sources as it claims, as well as using up stores delivered before the cutoff, diplomats here said. But they cautioned that it might have serious supply and morale problems if, as generally expected, the Reagan administration fails to obtain more money for it early next year.

"The real story on the contras counterrevolutionaries is what's going to happen . . . when they see that they are on their own for good," a high-ranking diplomat said.

The rebels have improved their military performance by emphasizing traditional guerrilla tactics of using mobile units to stage hit-and-run ambushes and avoiding contact with larger, better armed government forces. They have stepped up attacks on farms and other economic targets this fall and currently are trying to disrupt the harvest of coffee, the country's principal cash crop.

Nevertheless, the guerrillas still lack the strength necessary to gain the upper hand in the war or to expand significantly their presence outside the remote mountain areas, the various military observers said. Moreover, they risk alienating peasants by sabotaging the economy and such persistently reported practices as robbing farmers and killing wounded combatants.

The Army has reinforced its troops in the north with draftees, increased its use of artillery and employed elite Interior Ministry combat units against the guerrillas, senior Army officers said, adding that new Soviet-made Mi24 Hind helicopter gunships are to be used next year. On balance, most observers predicted that neither side would gain a decisive advantage in coming months.

The Popular Sandinista Army's senior field commander in the north said that the guerrillas were fighting a "war of attrition," and he predicted that the Army would be in "a better position" only in late 1985, when he hoped that several roads would be completed penetrating the rugged terrain frequented by the rebels.

"We're hoping that with each offensive that they mount, our military strength will be greater," said commander Javier Carrion of the Third Military Zone in an interview at his headquarters in this provincial capital 79 miles northeast of Managua.

The northern front always has been the most important because the guerrillas are strongest here and are closest to the country's main population centers. But it has become even more critical this year as fighting gradually has slowed along Nicaragua's southern border with Costa Rica and along the Caribbean coast.

The FDN's principal change in strategy has been to give up attempts to seize and hold a slice of territory. It found that it could not take on the Sandinistas in large numbers, particularly when the Army brought up Soviet-supplied mobile rocket launchers and howitzers.

The FDN has settled into a classic guerrilla strategy of spreading its forces in rough terrain, frustrating the Army by avoiding large confrontations, harassing the economy and trying to build civilian support.

"The tactics they use are pretty good. They make us run around a lot chasing them and do not confront us directly," 2nd Lt. Enrique Bucardo said at his command post in the northern town of Yali.

While the rebels are fighting "smarter," as one foreign military source said, it was unclear whether they are as strong as before the funding cutoff. Carrion said that the rebels' offensive in October, aimed at disrupting the Nov. 4 elections here, was not as intense as the one last summer and that Army casualties had dropped during the past three months.

Some observers said that the guerrillas simply were targeting the economy more and the Army less, while others suggested that they were running short of ammunition and other supplies.

The rebels' strongholds are in the north of Jinotega province -- particularly north of the small towns of Yali and Pantasma -- and in the northeastern part of neighboring Matagalpa province. Traveling in "task forces" of up to 300 men, however, they have penetrated regularly into neighboring provinces and appear to be covering more territory than in the first half of the year, government officials acknowledged.

"We don't deny that there are sectors where they have influence, but they are only marginal areas in the mountains where there aren't any roads," Carrion said. "Our troops go there, show a presence, and the FDN withdraws to Honduras. Since we have our own material and technical difficulties maintaining troops in the mountains, the FDN returns cyclically."

Estimates of the rebels' strength vary. The Army says that the FDN has between 6,000 and 7,000 men under arms, discounting the force's own claim of 15,000. "We haven't seen 15,000," Carrion said.

The force's most significant change in tactics this autumn has been its increased attacks on the economy, including raids for the first time on privately owned farms in addition to state-owned or state-financed cooperatives.

Out of 89 attacks on state farms and cooperatives in Jinotega and Matagalpa provinces since the war began in 1982, 22 have occurred in the past three months, said Miguel Barrios, the government's chief agriculture official for the two provinces.

He said that the guerrillas began to raid privately owned farms -- six reportedly have been attacked since September -- because private owners lacked resources to rebuild and the damage thus was more lasting. In a typical raid, a guerrilla task force overwhelms a much smaller militia force defending a farm, burns its buildings and equipment, and kills militiamen, local Sandinista officials and known government sympathizers.

Officials and residents of several northern towns also charged that the rebels regularly kidnap young men to fight in their ranks and women to carry backpacks and cook. It was not confirmed that this was happening.

The rebels also regularly ambush trucks carrying food or other supplies, and, to a lesser extent, trucks or buses carrying passengers. These vehicles normally carry several armed militiamen because of the danger of attack.

Facing such guerrilla tactics, the Sandinista Army has responded with classic counterinsurgency tactics. From its large bases in provincial capitals, it has established firm control of the roads and sends out regular patrols to hunt the guerrillas in the mountains.

Many of the infantrymen on these patrols are from the more than 20,000 young men inducted since a draft took effect a year ago.