The Reagan administration, increasingly concerned that the Chilean military government's crackdown on dissent could trigger widespread confrontation and violence, is sending its top Latin America expert to urge President Augusto Pinochet to resume a dialogue with political opponents on ways to restore democratic rule.

The State Department announced yesterday that Langhorne A. (Tony) Motley, assistant secretary for inter-American affairs, will travel to Santiago today "to review the current state of U.S.-Chilean relations and to convey to the government, the church and democratic political leaders U.S. views on a range of issues . . . including United States support for a return to democracy in Chile."

Motley's trip is the latest expression of U.S. concern that Pinochet's repressive tactics could be leading to an upheaval that would give radical leftists a chance to regain the power they lost in 1973 when Pinochet overthrew Marxist President Salvador Allende in a bloody military coup.

U.S. officials are particularly anxious to guard against Chile's becoming a South American version of Nicaragua, whose Sandinista government is blamed by Washington for supporting communist subversion in Central America.

The administration initially followed a policy of cultivating good relations with Pinochet and nudging him toward accommodation with Chile's moderate opposition on a plan for gradual return to elected civilian rule. However, after tentative moves toward flexibility, Pinochet abandoned the idea of dialogue and instituted a campaign of repression against the widespread strikes and leftist violence that have broken out in the past year.

Adding to the tensions has been a growing feeling within Chile, reportedly shared by Pinochet, that the United States has lost confidence in his ability to control the situation and is seeking ways to force him out.

Chilean sources say that that impression has been heightened by a U.S. abstention last week when 12 other countries voted for an Inter-American Development Bank loan to Chile and by sharp criticism of Chile in the State Department's latest report on international human rights.

Secretary of State George P. Shultz also is ousting U.S. Ambassador James Theberge, an advocate of supporting Pinochet, and plans to replace him with Harry G. Barnes Jr., a career diplomat regarded as more responsive to Shultz's direction.

U.S. officials dealing with Latin America insist that they are not trying to overthrow Pinochet. They stress that, despite signs of misgivings among younger Chilean officers about the wisdom of Pinochet's course, his control over his armed forces remains strong while the United States has virtually no influence with the Chilean military.

In addition, these officials say, U.S. hopes for Chile would not be served by an abrupt change of leadership that would only heighten tensions and give the radical left an opportunity for increased support.

The officials said that Motley's purpose would be to make that clear and to try to start a process of convincing Pinochet and the moderate opposition parties that they all have a stake in finding a plan and a timetable for a step-by-step return to elected civilian government.