The International Committee of the Red Cross has "made it clear that it is prepared to pull out of El Salvador" because of growing concern over the Salvadoran armed forces' practice of not taking prisoners in battle, according to a confidential State Department cable to the U.S. Embassy here.

The Red Cross' departure "would be a severe blow to our efforts to maintain congressional support" for U.S. policy in El Salvador, the cable said.

Signed by Secretary of State Alexander M. Haig Jr., and sent to the embassy in May, the cable instructs diplomats to seek "concrete" actions by the Salvadoran government on a range of problems, including that of the prisoners, that might cause the Reagan administration difficulty in its efforts to certify to Congress that El Salvador is qualified to continue receiving U.S. aid.

The certification that, among other things, the Salvadoran government has made progress in improving human rights conditions here, must be submitted by the administration at the end of this month.

The cable asks for evidence of progress "which we could use in defending certification" necessary for continued assistance. The confidential document, whose authenticity was confirmed by a U.S. Embassy official here, was distributed this week to U.S. journalists under a cover letter of an organization called the Central American Information Office. Calls today to the Cambridge, Mass., telephone number listed were answered by a tape recording asking the caller to leave a message.

A member of the Red Cross delegation here, Andres Balmer, said any decision to withdraw the mission would be made at Red Cross headquarters in Geneva, Switzerland. The Red Cross mission here visits Salvadoran prisons, monitoring compliance with Geneva Convention standards for treatment of prisoners.

Balmer said that virtually all of the approximately 550 political prisoners held by the Salvadoran government were arrested in noncombat situations and that there never have been more than a few captured on the battlefield during the country's 18-month-old civil war.

"There was recently a large military operation in Chalatenango," Balmer said, referring to a government offensive last month involving the three U.S.-trained rapid reaction battalions in the northern province. "The Army announced that 130 had been killed in the fighting, but there was not one prisoner. That worries us a lot.

"We are here to provide humanitarian aid in a zone of conflict. It is not our role to protest; we never make public condemnations" about situations monitored by the Red Cross, he said. "By protesting or pulling out we would lose that effectiveness. On the other hand, we do not intend to allow our presence here to become a label the government can point to to demonstrate its compliance with humanitarian norms."

Relations between the Red Cross and the government were strained two weeks ago when the Red Cross mission accepted a public offer by guerrilla forces to take charge of 43 Army prisoners captured by the guerrillas in fighting in Morazan province. The Salvadoran government failed to respond to a Red Cross request for safe conduct to bring the prisoners out of the battle zone, a Red Cross official said, and nothing has been done to carry out the transfer.

A diplomatic source here said the Red Cross first indicated it would consider pulling out the mission after a Salvadoran official, addressing the U.N. Human Rights Commission in Geneva last March, cited the Red Cross' presence in El Salvador as an indication that human rights are being respected. The United States has avoided any such interpretations of the Red Cross' presence in its human rights reporting.

Balmer said he has toured military installations around the country, giving talks to troops about the Geneva standards and encouraging them not to kill prisoners. But he said soldiers and their officers admit that they routinely kill prisoners, because it is considered too dangerous to transport them from the battlefield.

"You would expect to find some prisoners sometimes in Chalatenango or Morazan [the areas of heaviest fighting], but nothing, never," Balmer said.

The State Department cable said that "we need to demonstrate immediately an improvement" in handling of captured insurgents and that "a specific order to take and protect prisoners issued and publicized by the Ministry of Defense should be highly desirable."

The cable suggested that the Salvadoran government invite "the ICRC [International Committee of the Red Cross] to assist in a program designed to ensure proper treatment of prisoners . . .The ICRC has made it clear that it is prepared to pull out of El Salvador if its concerns are not met."

The cable also recommended a "material incentive system to ensure that prisoners are not killed." The system would call for Army intelligence officers to pay a "nominal sum" to paramilitary units to turn in prisoners they have captured. The U.S. Embassy official said today that nothing ever came of that idea.

On the issue of prisoners held by the guerrillas, the Salvadoran government apparently has been reluctant to allow the Red Cross to arrange releases for fear that the transaction would increase the international stature of the Farabundo Marti National Liberation Front, implying the rebels have rights as "belligerents," according to Red Cross and diplomatic sources.

Guerrilla leaders have said their policy is to release prisoners they take as soon as possible, and Balmer said he is familiar with cases of captured soldiers who have returned after release to their military units. A year ago the guerrillas announced they were holding 11 prisoners in Morazan and asked the Red Cross to arrange their release. In that case also the government refused permission. The latest batch of prisoners was taken in heavy fighting in early June following the guerrilla occupation of the town of Perquin in northern Morazan. One of the prisoners the guerrillas claimed to hold is Col. Francisco Adolfo Castillo, the deputy minister of defense, whose helicopter was shot down in the Perquin area June 17.