President Daniel Ortega charged today that the United States was behind what he called an unprovoked attack by Honduran jets on Nicaraguan positions near their common border.

Government officials denied Honduran claims that one of Nicaragua's Soviet-built Mi8 helicopters had been shot down in the incident, which occurred yesterday morning near the Nicaraguan town of Boca de Poteca.

Both Honduras and Nicaragua appeared to be taking this incident more seriously than any of the dozens of other border incidents in recent months.

In an emotional speech to crowds observing Central American independence from Spain in 1821 and the ouster of the American adventurer William Walker from Nicaragua about 30 years later, Ortega warned that the Honduran Army is "concentrating troops in order to launch new attacks on our country." He said the actions "are not of the free will of the government of Honduras but are imposed by the United States."

Ortega invited Honduran President Roberto Suazo Cordova to meet with him to discuss the two countries' differences.

In Washington, the State Department issued a strong statement denying any involvement in the attack, and saying, "We hold the government of Nicaragua fully responsible for this escalation of the level of violence in Central America."

The statement, by spokesman Joseph W. Reap Jr., said the Honduran "action against a Sandinista military target was in direct response to a Sandinista attack against the territory of Honduras." It said the United States supports "the exercise by the government of Honduras of its legitimate right of self-defense."

The Honduran government said last night that its planes had attacked artillery positions in Nicaragua after Sandinista mortars had fired into Honduran territory, killing one person and wounding eight others.

Suazo Cordova denounced "this unheard-of warlike attitude" and "the irresponsible, bellicose expansionism of Nicaragua." He called the Honduran Congress into emergency session, recalled his ambassador to Nicaragua and declared a military alert along the border.

But diplomatic sources here said that Honduras has indicated that it intends no further military action at this time as long as Nicaragua holds its fire. The sources said the Honduran government had apparently decided in the past 24 hours that the constant border incidents had to be stopped and that it would respond strongly to the next one and to others in the future.

Nicaragua has maintained that all its border activity is in response to attacks from U.S.-backed rebels, known as contras or counterrevolutionaries, who are fighting to overthrow the Sandinista government. In a communique last night, Nicaragua said 800 contra "mercenaries" had crossed into Nicaragua at Boca de Poteca, "with the support of the Honduran Army and Air Force."

The diplomatic sources said there had been no reports of contra troops in the area where the incident occurred.

Bosco Matamoros, a rebel spokesman in Washington, said no rebel units stage raids across the border. He said all operate inside Nicaragua.

Sandinista military officials, asking that they not be named, said two Honduran Super Mystere jets had flown into Nicaragua to the town of Wiwili, 20 miles from the border, and had fired missiles at Sandinista helicopters without hitting them.