Nicaragua charged in the Security Council today that an additional 4,000 rebels are primed to cross the Honduran border in the next few days, at U.S. instigation, to join an insurgent drive aimed at ousting the Sandinista government. U.S. Ambassador Jeane J. Kirkpatrick termed the charges a "myth."

Nicaragua called for the U.N. debate to muster international concern over the fighting. Its deputy foreign minister, Victor Hugo Tinoco, said 2,000 rebel troops invaded Nicaragua three weeks ago but "have already been found and encircled."

The real danger, he warned, is not from these "diversionary actions" but the possibility that the Honduran Army and "other forces in the region" might strike along the Pacific Coast toward the capital, Managua.

Kirkpatrick responded by saying it is a "myth that Nicaraugua is about to be invaded by the United States or Honduras or someone."

She did not reply in detail to the Nicaraguan charges but sought instead to focus attention on reported human rights violations by the leftist Sandinista government and detailed evidence that it is increasingly totalitarian and militaristic.

She said that the insurgents, described by Nicaragua as former members of the forces of deposed dictator Anastasio Somoza, are "in a very large number of case democrats" who opposed Somoza but defected "when the Sandinistas became military dictators."

In coming to the United Nations, she charged, Nicaragua was seeking to assert the nonexistent "rights of repression of its own people and to direct revolutions against its neighbors."

She concluded that it is no wonder that "Nicaraguans thirsty for real freedom are now once more ready to fight for it. The right to repression sought here today cannot be granted by anybody."

Ambassador Enrique Ortez Calindres of Honduras denied that his government had attacked Nicaraguan outposts or plans an invasion.

He insisted that there "are no military camps on our territory" but conceded that there are 35,000 Nicaraguan refugees on Honduran soil. Ortez noted that it is as impossible to control the border now just as it was when the Sandinistas infiltrated across it during their own guerrilla operations before their 1979 victory.

The Nicaraguan warnings, he said, are a "prelude to war against Honduras."

The U.S. delegation had sought without success to delay today's debate until after President Reagan's speech tonight, which described Soviet military activity in the Caribbean and Central America.

The Nicaraguans made clear that they would have no resolution to propose after the long list of speakers is exhausted some time Thursday or Friday but were satisfied with the opportunity to dramatize their charges.

Tinoco said that according to Nicaraguan intelligence the infiltrators were planning to move forward in the next few days in the Jalapa region, toward gold mines in the northeast of the country and toward Puerto Cabezas on the Atlantic Coast. He said that Honduran troops were concentrating near the border along the Pacific Coast, but he never specifically accused the Honduran government--as opposed to the Honduran Army--of planning an invasion.

Tinoco conceded that the invaders have not yet been repelled but expressed confidence that they would be "defeated militarily in the near future."

News services reported the following:

The Sandinista government rushed more than 300 militiamen to the northern city of Matagalpa as heavy fighting against rebels based in Honduras continued, military sources in Managua said.

Combat continued in the El Dorado, Chachagua and Palo Prieto hills in Matagalpa province and near the towns of Quilali and Wiwili in neighboring Nueva Segovia province, the military sources said.

The rebels' Radio 15 de Septiembre claimed 293 government troops had been killed and at least 70 wounded at a cost of only four insurgents killed and four wounded in the past week of fighting.

Nicaraguan Defense Minister Humberto Ortega said at a news conference yesterday that 205 rebels, 57 government soldiers and eight civilians had been killed in the fighting, which began last month.

The government barred journalists from the combat area, about 110 miles north of Managua, so the rival claims could not be checked.

In San Ramon, 60 miles northeast of Managua, members of the Sandinista Army-organized People's Militia said they and regular soldiers had beaten the guerrillas in fighting during the past week. He denied rebel reports that they had seized the mountain town.

But the militiamen said fighting apparently was continuing to the east near the town of Rio Blanco and possibly south near San Dionisio.

The incursion by the guerrillas into Matagalpa was the deepest penetration by the rebels since they began sporadic attacks against the Sandinistas about three years ago, authorities in Managua said.