The Reagan administration believes that the level of opposition to Nicaragua's Sandinista government has become substantial and is continuing to grow, according to a senior State Department official.

The official, who met with several Washington reporters on condition that he not be quoted by name, called the growth of anti-Sandinista movements part of a new situation developing in Central America.

While saying that he is not predicting that the Nicaraguan government will be overthrown, the official said that the level of opposition to that government may not be very different from that in nearby El Salvador, where a bloody civil war has been raging for several years.

So far, only a few incidents of armed conflict have been reported in Nicaragua, several of them near the Honduran-Nicaraguan border, where anti-Sandinista forces are reportedly encamped. Some of the forces are reported to be receiving secret aid from the CIA under a $19 million program approved in November by President Reagan.

Assistant Secretary of State Thomas O. Enders, the senior State Department official monitoring events in Central America, charged in a speech last week that the Nicaraguan state is "the preserve of a small, Cuban-advised elite of Marxist-Leninists, disposing of growing military power and hostile to all forms of social life but those they dominate."

Enders also said, in an address to the Commonwealth Club in San Francisco, that neither the United States nor Nicaragua's neighbors challenges that "it is, of course, up to Nicaragua to decide what kind of government it has."

However, the senior State Department official, speaking to reporters, said there is a belief among its neighbors that Nicaragua, as presently constituted, may be imcompatible with the rest of Central America. This basic question will have to be faced in the future, he said.

The Reagan administration repeatedly has charged that the Salvadoran insurgents are being directed and aided from headquarters in Nicaragua. The Nicaraguan government repeatedly has denied this.

Enders, in his speech, reported that the administration has offered to help the Sandinistas locate the Salvadoran guerrilla headquarters, which Enders said had been located recently in a suburb of Managua. "Nicaragua has yet to respond," he added.

A Nicaraguan Embassy official said the United States proposed on July 2 to provide "experts" to assist Nicaragua in locating the Salvadoran headquarters. The official said that Nicaragua had made it clear on several occasions that it is prepared to discuss all U.S. points, including allegations of assistance to the Salvadoran insurgents, in the high-level dialogue that Nicaragua is seeking.

The State Department this spring announced U.S. willingness to enter into such high-level talks but the administration cooled to the idea within a few weeks. Oral and written messages have passed back and forth at the ambassadorial level, but no high-level discussion is in sight.

The senior State Department official did not rule out such talks, but said a next step would be to consider the attitudes of Nicaragua's neighbors toward a high-level U.S.-Nicaraguan meeting.