President Jose Napoleon Duarte of El Salvador told President Reagan yesterday that the leftist guerrillas fighting his government are growing weaker.

Duarte told reporters after meeting Reagan at the White House that he did not ask for any new military or economic aid, but said he is attempting through his 10-day trip to the United States to win additional government and private support for his cause.

Duarte said Sunday that he is seeking $300 million in economic aid from the international community.

Although White house officials called yesterday's meeting a private visit, a Marine guard held the flag of El Salvador outside the entrance to the West Wing lobby while Duarte was inside. After talking with Reagan, Duarte stayed on for a 15-minute conversation with Vice President Bush.

A White House official who asked not to be identified said Duarte thanked Reagan for the economic and military aid already supplied to his government. The Reagan administration has committed $109 million in economic aid and $35 million in military aid to the Duarte government for 1981.

Duarte gave Reagan a status report on the El Salvadoran civil war, the U.S. official said.

Duarte told reporters that his opposition is getting weaker and said that he is looking for a way to end the fighting. "We are eager to find a solution, a political solution. We're not waiting for a military solution," he said.

In response to a question from Bush about reported acts of violence against civilians by the government's National Guard, Duarte said 600 National Guardsmen had been dismissed and 64 are in prison for such acts. He said more than 20 officers had been dismissed, the U.S. official said.

Duarte said he has urged all forces in El Salvador to renounce violence as a first step toward seeking a political solution. "I believe that my people are ready to look for a democratic solution," he said.

The U.S. official said Duarte indicated that if the leader of the leftist opposition, Guillermo Ungo, would abandon violent tactics, the two sides could then negotiate with the aim of arranging elections.

Duarte's assertion that his internal foes are getting weaker seemed to contradict the assessment of the Reagan administration. At a press conference three weeks ago, Secretary of State Alexander M. Haig Jr. said the main targets of the insurgents are civilians and the State Department, through spokesman Dean Fischer, sounded an alarm over increased violence in El Salvador.

El Salvador has been among the most controversial foreign policy issues of the Reagan administration. When the administration first voiced alarm over the civil war and moved to bolster Duarte's government, it aroused fears that the United States was going to become deeply involved in the civil warfare.

However, Reagan ruled out sending U.S. combat troops to El Salvador. There are about 40 U.S. advisers there helping to train government forces.

After meeting Duarte, Reagan spent about two hours working on the draft of the speech he is expected to give this week on his new round of budget cuts. The speech most likely will be tomorrow night.

The president began his day with a breakfast meeting with business and civic leaders at which he urged that the private sector take over "things that are not being very well done by government." Reagan said he intends to make a major speech to the business community on this theme soon.