With United States pressure, advice and money behind it, what may become the most sweeping land reform in Central American history is now under way in this violently divided country.

Combined with the nationalization of the country's banks and foreign commerce, it is the beginning of an effort to restructure the nation socially and economically to gain crucial popular support for the U.S.-backed military-civilian coalition government -- and to bring peace.

But despite, and many San Salvadorans believe because of, the government's declaration of a partial state of siege, the killing continues. An average of 10 persons a day are now dying in political violence, with more than 700 killed since the beginning of the year, according to Catholic Church officials.

Described by one land reform official as "the main laboratory for U.S. political experiments in Lation America," El Salvador is still threatening to blow up.

Extremists on the left continue to mount attacks on the military and civilians believed to be informers. The extremists on the right have retaliated with a vengeance.

Although thousands of peasants are already participating in the land reform program, there are hundreds who do not claim the fields the government is redistributing, but come to the city to take refuge from the violence.

Leaders of the military-civilian government deplore the violence. Minister of Defense Col. Jose Guillermo Garcia expressed on television Tuesday night the Army's "total support" for the reform and condemned both left and right extremists as "savages." But many Salvadorans believe that elements of the armed forces are responssible for much of the oppression.

According to informed diplomatic and civilian sources, El Salvador's wealthy, conservative elite has recently hired defeated troops of Nicaragua's National Guard and men from Guatemala as mercenaries. These are believed responsible for some of the efficient murders of leftist political leaders, including members of the current regime.

The failure of Salvadoran security forces to move effectively against terrorists on the right and the military's reputation for ruthlessness have not created confidence in the reform program promoted and backed by the armed forces.

Troops accompany social worker and technicians onto coffee, cotton and sugar plantations to announce their seizure and redistribution to the peasants.

The stated reason for the Army's presence is that it is there to protest the government workers and peasants from attack by extremists. But many Salvadorans, including influential Catholic Archbishop Oscar A. Romero, fear that the policy will result in only more oppression.

"The worst aspect is that it could bring about the systematic militarization of the whole republic," said Romero.

This same belief has caused deep divisions in the Christian Democratic Party, which originally joined young military officers in the five-man governing junta at the beginning of the year.

But many liberal Christian Democrats, including junta member Hector Dada, have resigned from the government and their party and left the country.

Most of those who withdrew their support objected to the lack of communication between the government and the so-called "popular organizations." cThese Marxist groups include many peasant organizations, but have not been visibly appeased by the reforms and have not participated in them.

The supporters of the land reform say that the new collectives will be open to any organized peasant group. They say the troops will leave the haciendas as soon as they have been secured. But aside from political problems, the reforms also suffer serious technical and financial deficiencies in these early stages.

Although $50 million in U.S. economic aid is on the way to aid the reform programs, they are still starved for money. On one farm near the capital 700,000 baby chicks are dying because the government cannot pay for medicine.

U.S. technical advisers were asked to help plan the reform, but now that the land is being taken over, there is a shortage of managerial expertise on several farms. It is a crucial question whether the previous farm management can be persuaded to stay on.

The land reform is planned in several phases. At present, only farms larger that 1,200 acres are being seized. Roughly 20,000 acres were taken in the first week with the total expected to double as about 400 separate properties are seized.

Several land reform officials say that they do no expect the next phase -- seizure of properties from 370 to 1,200 acres -- to take place for at least another year.

With continuing violence and many technical problems to overcome several officials said they would be satisfied just to see the first phase completed successfully.

As one official put it after spending a half-hour charting the intricacies of the reform on a blackboard: "Everything you see is a big maybe."