The Soviet Union and its allies are funneling military equipment into Nicaragua at about double the rate of the past two years, according to U.S. officials.

American specialists say they believe that the speedup in Soviet deliveries is due in large part to stepped-up actions by U.S.-backed rebels fighting inside Nicaragua and long-standing Nicaraguan concerns about a possible conflict with Honduras, whose territory to the north served as a launching point for some of the rebel forays.

Much of the newly arriving equipment appears intended to make Nicaragua's 25,000-man regular army much more mobile than it is now. Specialists here say they believe that army is going to have to move out into the field much more quickly and more often than it now can if it is to challenge and contain separate rebel forces now operating in the northern, eastern and southern regions of the country. Such challenges will mean long supply lines.

The rebel forces, which have been growing in strength in recent months and have stepped up forays throughout the country, are estimated at more than 9,000 guerrillas, U.S. officials say.

The anti-government force fighting inside Nicaragua is now bigger than the roughly 6,000-man, Cuban-backed force fighting troops of the U.S.-supported government inside El Salvador. And the new shipments of Communist-bloc equipment to the Sandinista leadership in Nicaragua reflect the increasingly similar challenges confronting governments in both countries.

The United States, in an effort to get Salvadoran troops to challenge the guerrillas in their strongholds, has sent helicopters and trucks to El Salvador.

Now, officials here report the Soviet Union has recently shipped eight to 10 Mi8 troop-carrying helicopters to Nicaragua and more appear to be en route.

Until now, the Nicaraguans are believed to have had only two of these troop transports.

In May, a Soviet vessel unloaded about 350 trucks in Nicaragua, reflecting about a one-third increase in the roughly 1,000 trucks from East Germany previously delivered.

In addition, officials say 20 to 25 Soviet-built BRDM2 armored fighting vehicles and BTR armored personnel carriers have arrived in recent months, along with BM21 multiple rocket launchers, ZIS2 57 mm anti-tank guns and a handful of additional tanks to boost the Sandinista total to about 60.

All told, officials here estimate that Soviet-bloc deliveries to Nicaragua are now running at a rate of about 20,000 tons a year, in comparison with about 10,000 tons annually in 1981 and 1982.

How much good this equipment may do the Nicaraguans is hard to predict, officials here say, because the government's regular army has not really been tested in battle in a guerrilla war that has only begun to develop real intensity in the past year.

Many of the clashes are with less well-trained local militia units.

Analysts here say, however, that some increasingly bold guerrilla activities in the past three months or so may have shaken up the Sandinista leadership and the Cuban military high command, which is heavily involved in guiding the Sandinista forces.

The Reagan administration, for some time, has claimed that there are about 2,000 Cuban military and security advisers in Nicaragua. The Cubans claim they only have "dozens" of such advisers.

Officials here say that in recent months there has been sharply increased air traffic between the two countries, and it is possible that as many as 1,000 more Cuban military advisers may have arrived, although they stress that they have no hard evidence of this.

They say they believe some increase is likely, however, because of the stepped-up threat from the contras plus the influx of more equipment to operate and maintain.

Both Pentagon and State Department specialists say they believe, however, that Cuban advisers are now spread down to field-level positions with Nicaraguan troops.

It is also known that a top Cuban general, Arnaldo Ochoa Sanchez, has been in Nicaragua for several weeks and is believed to still be there.

Ochoa was the top officer sent by Cuba to Angola and Ethiopia in the 1970s prior to a large buildup of Cuban troops in those countries. His arrival has touched off speculation that a large Cuban buildup may also be in the offing for Nicaragua.

American officials say they believe that the contras have hurt the Nicaraguan forces and that casualty levels are already taxing limited Nicaraguan medical facilities.

These officials say the guerrillas are in no position to defeat the government's forces or overturn the government, and that such an outcome is unlikely and, if it were possible, would take years.

Still, they describe the contras as having achieved a considerable success, measured by guerrilla standards, in having established themselves in about six months as a force to be reckoned with and not just a hit-and-run operation out of Honduras.

But where it goes from here, officials say, depends in large measure on how the Nicaraguan army fights and that may be what Gen. Ochoa is trying to assess and why more Cubans may be arriving to stiffen control of the army.

Although the military action appears to be intensifying, American officials looking at the whole picture say they believe that the search for a peaceful solution in Central America, beginning in El Salvador, is a few steps closer now than it was last year.

They cite the recent comments of the just-departed U.S. ambassador to El Salvador, Deane R. Hinton, who called attention to the "progress" represented by the creation of an official Salvadoran Peace Commission to explore talks with leftist guerrilla forces and the setting forth by the guerrillas of an agenda for such talks, including some points that might be acceptable to the government.