The United States expressed deep concern yesterday about reports of atrocities committed by Nicaragua's National Guard against unarmed civilians and urged President Anastasio Somoza to "discipline and control" his troops.

A public statement issued by the State Department also called for urgent investigations of the reports by the Nicaraguan government and by the Inter-American Human Rights Commission.

The commission is scheduled to go to Nicaragua on Oct. 5 to look into charges of rights violations, but the State Department suggested that, in view of the new allegations, it should make the trip earlier.

U.S. officials said the statement was prompted, in part, by a report in yesterday's Washington Post quoting people in the Nicaraguan city of Leon as saying that 14 young men were executed by the guard last Friday as they pleaded for mercy.

Since Somoza has denied that his forces have committed atrocities during the civil war racking his tiny Central American country, the statement seems likely to widen the increasingly apparent rift between him and the Carter adminstration.

In response to questions about U.S. relations with Somoza. State Department spokesman Thomas Reston repeated earlier assertions that the adminstration is not seeking the overthrow of Somoza or any other government leader. But, Reston did say the United States holds the Nicaraguan government, which is under Somoza's dictatorial control, responsible for the actions of the armed forces.

"We are deeply concerned by mounting reports of atrocities committed against unarmed civilians by personnel of the Nicaraguan National Guard," the statement read by Reston said.

It added: "The government of the United States urges the government of Nicaragua to conduct an urgent investigation and to discipline and control its military forces . . . Our ambassador in Nicaragua has expressed our concern to President Somoza and he will continue to do so".

The U.S. statement came as the Organization of American States prepared for a foreign minister-level meeting here today to discuss the Nicaragua situation. U.S. officials said Deputy Secretary of State Warren M. Christopher will represent the United States at the meeting in the absence of Secretary of State Cyrus R. Vance.

The OAS meeting is part of a multipronged effort, strongly supported by Washington, to find some means of mediating the conflict between Somoza and his domestic foes. Somoza, whose family has controlled Nicarargua for 45 years, is under attack from a broad array of opponents ranging from leftist guerrillas to important segments of the Nicaraguan business community and the Catholic Church.

U.S. officials said yesterday the National Guard appears, at least for the moment, to have crushed most of the armed opposition to Somoza. But, the officials added, new fighting can be expected once the guerrillas regroup in the Nicaraguan countryside, and Washinton believes the need for mediation remains urgent.

Despite Washington's repeated denials that it is seeking Somoza's overthrow, it's an open secret that the administration believes he should step aside as the best hope of ending the turmoil.