Nicaragua's revolutionary government agreed yesterday to begin negotiations on easing tensions with the United States generated by the civil war in El Salvador, and it called on the State Department to fix a date for the start of talks.

In a meeting with Thomas O. Enders, assistant secretary of state for inter-American affairs, Nicaraguan Ambassador Francisco Fiallos Navarro said his government was willing to discuss all issues, including the eight-point proposal offered by the United States last week as a basis for improving relations.

State Department officials said only that the Nicaraguan response to the U.S. plan had been received and would be studied carefully. However, diplomatic sources said the Nicaraguans had expressed their willingness to begin talking as soon as possible, preferably with Mexico as the site, and had called on the United States to set a time and appoint its negotiators.

At the heart of the dispute is the U.S. contention that the radical, Sandinista-controlled Nicaraguan regime has been providing arms, guidance and other aid to the leftist guerrillas seeking to overthrow the authorities in El Salvador. The core of the U.S. proposal, made last Friday, involves an offer to resume American economic aid if Nicaragua halts its assistance to the Salvadoran insurgents.

The United States, responding to intermediary efforts by Mexican President Jose Lopez Portillo, announced last month that it was willing to enter into talks with Nicaragua. However, progress toward the bargaining table had been held up by uncertainty about the composition of the new government being put together in El Salvador and by the preoccupation of Enders, who is expected to head the U.S. negotiating team, with the more immediate Falkland Islands crisis.

According to the diplomatic sources, Nicaragua, in its response yesterday, said it is willing to undertake "formal and serious negotiations . . . to overcome the current crisis." In keeping with its past position, it did not acknowledge any involvement in the Salvadoran conflict and it warned that it rejects the idea that Central America is "a geopolitical reserve of the United States or a part of its so-called strategic frontiers."

However, the sources continued, although the Nicaraguan message contained a lot of rhetoric about that country's determination to preserve its independence in the face of outside pressures, it also stated that there is "no objection to discussing the eight points of the U.S. proposal or other points that the United States deems appropriate."

Nicaragua, the message said, "is ready to improve the climate of relations based on mutual respect and no conditions about the right of our self-determination."

The Reagan administration privately has taken the position that, while it will keep an open mind, it does not hold much hope for success and is pursuing the negotiations route as a concession to Mexico and to protect itself from charges by congressional critics that it is interested only in a military solution to the Salvadoran conflict.

In addition to calling for Nicaragua to end its help to the Salvadoran rebels, the eight-point U.S. plan proposes steps to ease Nicaraguan fears about the activities of anti-Sandinista exiles in the United States; pledges by the two countries not to interfere in each other's affairs; limitations on weapons, armed forces and foreign military advisers in Central America; a system of verifying that the arms traffic in the region has stopped, and a request to Congress to renew currently suspended economic aid to Nicaragua and make it eligible for the trade and investment benefits of President Reagan's Caribbean Basin initiative.

Despite Nicaragua's apparent willingness to help ease tensions, Managua last night extended for another 30 days its month-old state of emergency that suspends most constitutional rights, charging that Nicaragua still faces the threat of an invasion, United Press Iternational reported.