Negotiations between the Colombian government and the leftist M-19 guerrillas holding 14 ambassadors hostage inside the Dominican Republic Embassy were expected to begin Sunday as the siege entered its fourth day.

The government agreed to the negotiations last night after the guerrillas reportedly threatened to execute two of the ambassadors. The guerrillas said they had already selected them but their identities were not revealed. Among those hostages is U.S. Ambassador Diego C. Ascenio.

The government agreed to hold the negotiations in a truck or van to be parked in front of the embassy, situated in a middle-class neighborhood near the National University.

There were unconfirmed reports that the guerrillas and the government were still discussing a guerrilla demand that dozens of military police in the immediate area be withdrawn. Other details of the negotiations, including who will represent the Colombian government, were still being worked out today, according to Martha Montoya, press secretary of President Julio Cesar Turbay Ayala.

It is believed that the guerrillas will be represented, at least initially, by some of the ambassadors being held hostage.

Estimates of the number of hostages still inside the embassy and the number of ambassadors among them have fluctuated daily since the take-over Wednesday.

Today, Montoya said the government's official count of ambassadors still held hostage is 14, from the Dominican Republic, the United States, Venezuela, Brazil, Mexico, Uruguay, Austria, El Salvador, Egypt, Guatemala, Haiti, Switzerland, Israel and the Vatican. The charges d'affaires of Paraguay and Bolivia also are being held.

Montoya said the government estimates that there may be 18 additional hostages. However, Olga Lucia Guzman de Olano, one of the female hostages released Thursday, said she is certain there were at least 57 hostages when the takeover occurred. She did not see all sections of the three-story embassy. Eighteen hostages have been released so far.

The released hostages said there were at least 25 guerrillas, more than double the number originally believed to have participated in the takeover. The guerrillas' leader has said he has at least 30 others with him and the State Department has said it believes 39 guerrillas are involved.

The guerrillas have demanded that Colombia free 311 political prisoners, that the countries whose diplomats are being held, pay a ransom of $50 million and that planes be provided for both the guerrillas and the political prisoners.

Both Panama and Costa Rica have offered asylum to the guerrillas on humanitarian grounds. However, it is thought unlikely that the Colombian government will agree to free the 311 political prisoners.

Bogota has remained quiet during the siege. There have been no demonstrations for or against the takeover, except for several hundred students from the university who gathered shortly after it happened to show their support.

Public opinion seems to be firmly against the guerrilas, although a number of Colombians have expressed admiration for their daring and some sympathy for freeing the political prisoners.

The embassy is ringed with specially trained military police, an anti-terrorist squad trained in part by French military advisers. Defense Ministry sources have said the soldiers are disciplined and trained for any eventuality.

The only official communique issued today came from the Red Cross, which announced that it had delivered a second truckload of food and supplies to the embassy.

Jorge Valdez, a roving ambassador from Mexico who negotiated the exile of Argentina's former president, Hector Campora, from his asylum in the Mexican Embassy in Buenos Aires, spent several minutes near the Dominican Embassy today.

Valdez is believed to be one of several diplomats from affected countries who could participate in negotiations.