Five weeks into a guerrilla offensive, the Nicaraguan National Guard has lost control of substantial portions of the country and, according to informed sources, is now waiting for President Anastasio Somoza to negotiate a political agreement in its behalf.

Sources both close to the Guard and within it said that there is now little feeling that lost territory can be regained, at least without substantial outside held.

The sandinista National Liberation Front, one source, said, "has some good options militarily. The cards are now in their favor." The guerrillas control most of northwest Nicaragua, including all border crossings into Honduras. While the guerrillas retreated last week from Managua under massive government bombardment, they control the important cities of Leon and Masaya and are besieging the Guard in Esteli and in the southern provincial capital of rivas.

Still, the source said, fighting here could easily continue for another month if a political solution is not reached.

Meanwhile, the Nicaraguan Red Cross repeated its appeal for emergency food assistance. Red Cross vice president Wilfred Cross estimated the Organization is now feeding nearly 200,000 people in Managua and said they had been unable even to begin large-scale relief efforts in other parts of the war-torn country.

Cross said a "desperate" transport situation last weekend had been somewhat alleviated over the past few days by emergency food flights from several Latin American countries and leasing by the International Red Cross of two planes.

Many countries, including the United States, have been reluctant to allow the use of military planes to airlift food, apparently for fear of being accused of ferrying arms to one side or the other. Charter companies, Cross said, "are all afraid to come here."

U.S. Ambassador Lawrence Pezzullo returned to Managua this morning after two days of consulatation Washington. Although Pezzullo last week asked Somoza to resign, both the Sandinistas and opposition political groups here rejected a U.S. plan for the transition of power.

Somoza reportedly agreed to resign, but only on conditions relating to the future of his Liberal Party and the National Guard. While Somoza reportedly was told that he was in no position to negotiate his departures, U.S. officials are known to be pressing a Sandinista-backed provisional government to expand its membership to include more moderate political and business sectors.

It was not known what variation of the U.S. transition plan -- which ignored the provisional junta and provided for several levels of interim government followed by elections -- Pezzullo brought with him from Washington or if he met today with Somoza. The United States Is believed to be symphatic to Somoza's concern for the National Guard. "He has a real feeling that loyalty is a two-way street and that he would be a cur to abandon them," a source said of Somoza and the U.S.-created army that has served his family for 45 years.

The Sandinastas have said they will not carry out Iranian-style executions of National Guard soldiers charged with "crimes against the people." The provisional junta has said that "decent" soldiers can be incorporated into a new Nicaraguan army and that all those accused of crimes will be turned over to an "institutionalized system of justice."

Although there is a widespread feeling that the government is a sinking ship, its propaganda continues to call for new National Guard recruits and to emphasize the Iran experience to any officer who may be thinking of a cease-fire.

Some high-level officers who believe further fighting would serve only to destroy the country feel any agreement reached with the provisional junta would not be respected by the Sandinista guerrillas.

The United States is concerned that at least a portion of the National Guard structure be retained to avoid a complete takeover by what it feels are radical elements within the Sandinista organization.

While a few weeks ago many government and Guard officials were still confident of a military victory, an informed source now says the National Guard was ill prepared, both from "inept intelligence" and outmoded strategy, for the strength of the Sandinista assault.

Although the government still has far more heavy armament than the Sandinistas, military analysts said its profligate use of ammunition, loss of garrison arsenals to the guerrillas and cutoff of foreign purchases has limited supplies.

"Nobody ever imagined the Sandinistas were going to be able to airlift" weapons and troops into the country, the source said. The guerrillas now are believed to have a number of planes, both small executive-type craft donated from private Nicaraguans and C47 transports that use small airstrips on private farms in northern Nicaragua.

The Sandinistas also have logistical problems, such as getting fuel to their planes and finding black-market arms, but the Guard's difficulties have been on a large scale.

Of its original four Shermans tanks, the National Guard now has only one. The guerrillas destroyed one, captured another and cannibalized a third for parts. Of 40 British-made armored vehicles the Guard used 18 months ago, only about 15 remain operational. Its originally small air force has been halved through mechanical problems or losses and at least one defection.

In northwestern Nicaragua, the Sandinistas now control all border points as well as a number of small towns and the cities of Leon and Matagalpa, where an estimated 100 National Guard soldiers evacuated the garrison early this week and are now surrounded, with no food and little ammunition, in the city cathedral. Several hundred National Guard troops in the surrounding hills have been unable to retake Matagalpa.

Most of the city of Estell is controlled by the Sandinistas, who have surrounded the National Guard garrison. The guerrillas also have surrounded Chinandega, while National Guard troops inside reportedly have armed Liberal Party civilians to fend off an expected assault.

Sandinistas are also reported to be battling for Rivas, 40 miles from the Costa Rica border.

Before the war began, a source said, Somoza's son, a National Guard lieutenant colonel who heads crack commando troops, "pleaded with his father to get [the Guard's] old museum pieces out of command positions" and replace them with younger, more aggressive officers. Somoza refused, the source said, because of long-time loyalty to the "old guard" and because he underestimated the Sandinista force. CAPTION: Pictures 1 and 2 Managuans at a checkpoint are ordered to demonstrate they carry no arms. At right, a looters' market. UPI