El Salvador's guerrillas, in a defiant response to President Reagan's speech last week urging an expansion of the U.S. commitment to the government they are fighting, have reaffirmed their determination to maintain ties to Cuba and Nicaragua.

In a broadcast last night, they also threatened "within that context" an "open regionalization" of their war if the Reagan administration continued to broaden its support for the faltering Salvadoran government.

In a broadcast over their clandestine Radio Venceremos, the rebels said: "We are and will continue being friends of the people and governments of Cuba and Nicaragua, and it does not shame us. Completely to the contrary, we are proud to maintain relations with those people--bastions of the anti-imperialist struggle. The Reagan administration is not one to tell the FMLN Farabundo Marti National Liberation Front who ought to be its friends and who its enemies." The statement made no effort to deny receiving Cuban and Nicaraguan support as the rebels have in the past.

The broadcast came as the Reagan administration is planning $110 million in military and $168 million in economic assistance this year to help launch some of the most ambitious counterinsurgency training and operations programs of the 2 1/2-year-old war.

Despite major questions about the competence and human rights records of the government here, Reagan justified the emergency support by pointing to the need to defeat "extremists with Cuban-Soviet support" that comes to them by way of "Marxist Nicaragua." He said the leftist forces would threaten U.S. security interests.

The rebels' broadcast defended their "right" to get arms anywhere. While insisting that their main headquarters are inside the country, along with their radio transmitter, they admitted to having "important missions" outside El Salvador.

"We have carried out important logistical operations of a clandestine character with which we have armed and munitioned our forces for a long time. We have carried out these operations by all the courses we could, and we have used all Central America and other countries for them," the broadcast said.

As Washington has raised its commitment in the region during the past month, the Nicaraguans also have reaffirmed their close ties, if not their concrete material support, with the Salvadoran rebels.

The Sandinista leaders in Managua feel under mounting pressure from a rebellion that reportedly receives covert funding from Washington on the basis that such action helps "interdict" arms supplies to the Salvadoran guerrillas. Speaking March 3 at a funeral for 17 adolescent Sandinistas killed by counterrevolutionaries, Nicaraguan commander Bayardo Arce warned that his party's "internationalism will not bend" and that "while Salvadorans are fighting to win their liberty Nicaragua will maintain its solidarity."

The guerrillas here said in their broadcast last night that their war "is and will continue being national, but we are not so naive as not to know that we cannot and ought not fail to place our plans in the framework of a regional conflict" in which the future of Central America is at stake.

They added that they are not closely tied to the Soviet Union but said that is because "unfortunately it is very far away."

The pace of the war here has stepped up dramatically since the guerrillas began a sustained offensive in October. The Salvadoran government and the White House maintain that the rebels use ammunition, if not guns, smuggled in from Nicaragua.

In a response that some military observers here see as virtually a last-ditch effort to reverse rebel momentum, the United States and the Salvadoran government are pushing ahead with an ambitious, coordinated program of military and civic action planned for some of El Salvador's battered, guerrilla-dominated eastern provinces.

Modeled broadly on the program of Civil Operations and Rural Development Support (CORDS) created in Vietnam during the 1960s, but refined and scaled down for implementation here, it is thus far referred to only as "the plan" and is to be carried out in four stages.

The number of U.S. military advisers required is not likely to remain below the administration's self-imposed limit of 55 men, according to a military observer. He added that the American presence in the countryside probably will grow with the addition of more U.S. Agency for International Development employes.

There is no set timetable as yet, but because of crop cycles in the San Vicente and Usulutan areas where the new initiative is expected to be launched, it should be under way no later than mid-summer.

According to military sources, the first stage, planning, is intended to integrate and coordinate the American and Salvadoran personnel working on the program as they hammer out its details. The second stage is a large-scale military sweep to clean out guerrilla concentrations. Salvadoran civilian assistance agencies trained to deal with everything from road repair to refugee assistance and public health are supposed to follow the Army's offensive. The plan calls for the military command staff to be advised by at least five American soldiers while another 10 to 15 are devoted to the task of "upgrading the training" of the troops in the operations area, a military source said.

In the third phase, the military role is supposed to subside while a renewed civilian infrastructure takes hold in these areas long dominated by the rebels. The fourth stage sees the withdrawal of all but a small contingent of soldiers while the main force moves on to new target areas. The strategy is designed to sidestep longstanding problems with senior Salvadoran military commanders who are ill-trained and ill-disposed to adopt the kind of political, social and military counterinsurgency tactics advocated by the U.S. Embassy.

Military sources here anticipate problems with the paramilitary forces needed to supply security in the latter phases of the program. Such groups have been responsible for many of the atrocities in the countryside that give this government a notorious human rights record.

There are also questions about how effectively the guerrillas can be cleaned out in the first place, since previous sweeps rarely have pushed rebels out of their stronghold for more than a few weeks.