Salvadoran soldiers battled throughout the afternoon today to hunt down a force of guerrillas who landed on a tourist beach from 30-foot boats in what U.S. Ambassador Robert White said could be seaborne aid from nearby Nicaragua.

The landing of about 100 fighting men of unknown national origin late yesterday at El Cuco in eastern El Salvador raised for the first time the serious prospect that the Salvadoran civil war could broaden into a regional conflict.

It follows persistent reports that Nicaragua's revolutionary government has been supplying arms to the rebels of El Salvador. These reports contributed to a U.S. decision announced today to resume military aid to the Salvadoran government.

[Reached by telephone in Managua today, Nicaraguan Foreign Minister Miguel D'Escoto "categorically and absolutely" denied any official involvement by his government in the Salvadoran conflict. D'Escoto said he had received no communications from El Salvador's government, which maintains an embassy in Nicaragua, concerning White's charges.]

[D'Escoto said he questioned "why representatives of other countries . . . that want to be involved" in El Salvador "should be the ones" to make charges of Nicaraguan involvement.]

The Salvadoran vice president and commander-in-chief, Jaime Abdul Gutierrez, said that by the middle of the afternoon the guerrillas from the boats had been surrounded and driven back to the beach where they had landed. No prisoners were reported taken.

Telephone calls to the area indicated that there was fighting last night and this morning. At one point, according to a National Guard commander in the nearby village of Intipuca, as many as 1,000 guerrillas were involved. It was unclear, however, whether these included gunmen from the boats or whether they were among the rebels who have been operating in the area for years.

"I believe the reports that approximately 100 men landed from Nicaragua yesterday at about 4 p.m.," White said this morning. Later, referring to the U.S. arms aid resumption, he added: "We can't stand idly by and watch the guerrilla movement receive outside assistance."

White described his information about Nicaraguan involvement as "circumstantial evidence, but it is compelling and convincng." He said the Salvadoran government had investigated the matter and found that the boats bringing the force ashore had departed from Nicaragua. But Salvadoran President Napoleon Durate and the country's military leadership did not publicly cite such specific intelligence.

According to Durate and the commanders, five boats about 30 feet long landed on the grey sands of the beach. The gunmen who disembarked started along the highway that leads to the city of San Miguel about 25 miles to the north, they added, but were intercepted by the Salvadoran Army in rocky hills not far from the beach.

The Salvadoran insurgents, in the fourth day of what they announced as a "final offensive," reportly are getting .50-caliber machine guns, recoilless rifles, 81-77 mortars and rocket-propelled grenade launchers from a number of sources, including Nicaragua. It was on the basis of these U.S. intelligence reports apparently that White -- who has consistently opposed major military aid to the government here -- changed his mind in the last several days.

"[The State Department said today that the aid had been restored because an investigation of the murders of four American women in El Salvador last month" is proceeding."]

[The decision, it said, had "also taken into account the current military situation in El Salvador."]

Last month White said, "If we escalate our military assistance we run the grave danger that countries sympathetic to El Salvador's left will escalate their assistance -- with the resulting grave danger that Central America and El Salvador in particular will become an international battleground between communist countries and the United States.

After returning from Washington this week, however, White said he believed the international left has already "upped the ante." This fit in with efforts by the Salvadoran government to emphasize what Durate describes as the "geopolitical" factors involved in the conflict here.

"Communist countries," Duarte said, are "intervening directly to support the guerrillas. We are collaborating to save the Americans from the geopolitical invasion of Marxism-Lenninism."

There has been talk among leftists in Mexico, Nicaragua and Costa Rica of organizing international brigades to help the Salvadoran rebels. Salvadoran leftists fought alongside the Sandinista army during the 1979 Nicaraguan revolution and public sympathy in Nicaragua -- reinforced by government propaganda -- is strongly in favor of the Salvadoran revolutionaries.

"[Nicaraguan Foreign Minister D'Eacoto said today that his country maintained "a policy of nointervention" in El Salvador and that "nobody should get involved"" there. "That includes everybody."]

[D'Escoto did not deny that some individual Nicaraguans may be taking part in the Salvadoran war -- both on the side of the guerrillas and in the form of soldiers of the former Nicaraguan National Guard, who fled the country following the civil war there, on the side of the Salvadoran government.]

[As for those helping the guerrillas, he said, the official nonintervention policy "does not mean we can prevent our people from expressing their feelings of solidarity" with the Salvadoran opposition.]

Partly as a result of government intimidation and partly because of distaste for the guerrillas, the left still has not found much support of the general strike it called for this week. The city and country are still functioning almost as usual.

The left is mounting sustained attacks in San Francisco Gotera to the distant northeast, in Zacatecoluca to the southeast, in Chalatenango to the north and Santa Ana to the west. But most of their fighting has been quick attacks and retreats designed to wear down the Army. That only two of the Army's 10 helicopters are still flying because the rest have been shot down or worn down under continuous use suggests a certain success for the guerrillas' hit-and-run tactics.

[The Salvadoran Defense Ministry said its troops had regained control of the provincial capital of Zacatecoluca and had repulsed the rebel attacks on Chalatenango, according to wire service reports. But fighting raged in San Francisco Gotera, where rebels trapped about 800 government troops late yesterday, the ministery said.]