National Guard troops began a major push this morning to dislodge rebel forces from the seatern half of the capital, where they have held a zone of several square miles for more than a week.

Laying down a barrage of artillery fire that echoed through the city and launching rockets from small propeller-driven and jet aircraft, government forces tried to penetrate dozens of cement-block rebel barricades.

The troops appeared to make little headway in the eastern barrios, where barricades are manned by youths of the "people's militia" with weapons ranging from high-caliber machine-guns to small pistols.

The militia is the urban arm of the Sandinista National Liberation Front guerrilla movement fighting to topple President Anastasio Somoza.

There are few regular Sandinista troops within Managua's "liberated zone," since most of them are fighting a much larger and more conventional battle with the National Guard near the southern border with Costa Rica.

At their headquarters behind the city barricades yesterday, a small Sandinista command force told reporters they consider themselves at "the doors of victory."

"If you stay a few more days, you should be able to accompany us," said Julio Lopez, leader of the United People's Movement that runs the Sandinista civilian support operations in Nicaragua's ubran areas. "We haven't yet passed massively to the final assault on the fort of the dictator."

That fort is known here as "the bunker," Somoza's office and residence within Managua's National Guard headquaters. Informed sources said the government believes that the bunker still is impenetrable and that the rebels cannot hold out in the barrios against massive National Guard firepower.

The guerrilla say, however, that "the conditions do not exist here for permanent war. The National Guard's infantry," Lopez said, "has shown itself incapable of fighting" the Sandinistas. "The only resource they now use are technical measures - aviation artillery and tanks."

The Sandinistas said they believed Somoza would leave only under force. "He's said it, and we believe him," said Carlos Nunez, a member of the nine-man Sandinista directorate.

In terviews over the past several days, Somoza for the first time admitted the possibility of losing his war against the popularly-supportted Sandinistas, but said he would go down fighting if he went down at all.

While he again accused the Sandinistas of wanting a Marxist government here, members of a proposed five-person provisional government named over the weekend by the guerrillas said in a Costa Rican press conference that they wanted a pliralist democracy for Nicaragua.

[Venezuelan Foreign Minister Jose Alberto Zembrano met with Somoza and then returned to Caracas without comment, Associated Press reported. He represented five Andean nations seeking a peaceful solution to the conflict.]

Sources close to the government said Somoza is trying, so far unsucessfully, to purchase more small aircraft for his Air Force. Only two of the government's four T-33 jet trainers are believed still in operation. Of ten Cessna rocket-equipped planes, three have been shot down and one was flown to Costa Rica last week by a pilot who deserted rather than follow orders to attack the northern city of Matagalpa.

Sandinistas who now occupy the city of Leon destroyed one of the governments's four Sherman tanks last week, a source said, and another tank was put out of commission when the rebels ambushed a government convoy. The rebels also captured an armored vehicle in their seige of the Leon garrison, in which two-thirds of the 150-man Guard contingent was killed or captured.

Sources said Sandinista troops fighting in other parts of the country are now believed headed toward the southern front, where 700 rebel troops invaded over the weekend. Under heavy rains that have impeded air attacks by the government, the two sides in the south are now reported in a standoff several miles north of the border.

The guerrillas were also reported yesterday to have attacked Corn Island, located 80 miles off Nicaragua's eastern coast, and to have taken a number of hostages in boats reportedly headed toward Costa Rica.

Meanwhile, tension grew in Managua as the guerrillas were believed ever closer, either out of advancing victory or desperation, to an attack on somoza's fortress. CAPTION: Nicaraguan President Anastasio Somoza in meeting with foreign reporters Monday. By Karen DeYoung - The Washington Post