A U.S. Army lieutenant colonel who was photographed Thursday carrying combat weapons in an area contested by leftist guerrillas has been ordered out of El Salvador within the week by U.S. Ambassador Deane R. Hinton, the ambassador said at a press conference this morning.

At least two junior officers who appear along with Lt. Col. Harry Melander carrying M16 automatic rifles in a Cable News Network videotape have been given "a firm oral reprimand," but are likely to remain on duty here, Hinton said. There were, according to Hinton, some extenuating circumstances in the case.

"Our intelligence indicated it was reasonably safe," the ambassador said of the El Delirio site where the advisers were supervising the construction of a prefabricated bridge to replace one blown up by guerrillas fighting to overthrow the U.S.-backed government here.

When asked why, if the area was safe, the soldiers were carrying combat weapons, Hinton said, "I wish that I knew. I wish that it had not happened this way, and they wish it hadn't happened this way."

Melander is one of the highest ranking U.S. military men stationed here and a member of the embassy's Military Group.

One photographer who visited the area around El Delirio Thursday said there was a guerrilla checkpoint on the road only a mile south of the site where the advisers were seen.

Asked about this report and just how carefully the scene was checked beforehand, Hinton said, "as carefully as could be done." But U.S. Embassy staffers here have said several times in recent weeks that they generally have very poor firsthand information on what is going on in the countryside at any given moment.

"There, of course, are no guarantees that something can't happen anywhere in this country," Hinton said. "But we took very much care before they went into the area and, as a matter of fact, there were no hostile actions."

Hinton said, "The officer involved Melander , who very much regrets this incident, was concerned for his own safety, but, lamentably, failed to discuss his concern either with the Military Group commander or with me."

When asked if U.S. personnel were involved in offensive combat activities here Hinton said, "Categorically, no."

The ambassador's remarks came as he sat beside U.S. Sens. Patrick J. Leahy (D-Vt.) and Claiborne Pell (D-R.I.) in the heavily defended U.S. Embassy here.

Leahy and Pell ended a two-day visit to El Salvador in which they made it clear to reporters and Salvadoran officials that continued U.S. aid to this country is being seriously questioned by Americans gravely concerned about the prospect of an ever-escalating war.

Pell, a member of the Senate Foreign Relations Committee, said that he favors "discussions and negotiations" with guerrilla leaders as the electoral process continues here toward national voting for a constituent assembly next month. One of the members of Pell's staff said both senators hoped to meet with the guerrilla leadership "at some point," but Pell denied that such a meeting would take place in Managua, Nicaragua, their next stop. In Nicaragua, the two senators are scheduled to meet with the ruling junta.

Leahy, meanwhile, described his conversation with the Salvadoran defense minister, Gen. Jose Guillermo Garcia, this morning as "a bit of an argument," and said Garcia's answers to questions about human rights abuses were "totally unsatisfactory."

Pell had described Garcia as saying efforts were under way to control such abuses by the Salvadoran Army and other security forces but that such efforts had met with little success despite the Army's good faith.

But Leahy said that with growing reluctance in the United States to support involvement in foreign conflicts the nature of America's vital interests in El Salvador would have to be shown more effectively than Garcia seemed willing or able to do.

"I happen to have gotten on very, very badly with Gen. Garcia this morning," said Leahy, who is a member of the foreign operations subcommittee of the Senate Appropriations Committee.

"The general cannot take a close-the-door attitude" if he wants to demonstrate good reasons for U.S. support, Leahy said. "Certainly if he is not going to be honest to visiting members of Congress, he is not going to get that support."

Leahy said he did not claim to be an instant expert on El Salvador, but said, "I don't like coming all this way to have a canned presentation given to me by Garcia in a videotape that makes me think of the old training films they used to hand out to district attorneys so we'd know how dangerous marijuana was."

Leahy and Pell also met with Salvadoran President Jose Napoleon Duarte, as well as other political and church leaders.

[In another development, CBS television quoted an unnamed Salvadoran guerrilla leader as saying 20 civilians killed by government troops in the San Salvador suburb of San Antonio Abad two weeks ago were part of a network of guerrilla supporters. The guerrilla leader reportedly said that the people were taken from their homes and shot by troops who appeared to have a very accurate list of the targeted civilians.]

[Original reports and eyewitnesses to the incident, in which the number of civilians killed has been estimated at between 19 and 28, described the victims as unconnected to the guerrillas. The Salvadoran government has described the killings as the result of a fire fight between military and guerrilla forces, although U.S. spokesmen have said they do not believe a fire fight was under way.]