The Pentagon's senior official on Latin American affairs will travel to Chile next week for a visit that administration sources yesterday characterized as part of a stepped-up U.S. effort to "engage ourselves seriously in the process" of moving Chile's 11-year-old military dictatorship toward democracy.

Officials said Deputy Assistant Secretary of Defense Nestor Sanchez would travel to Santiago March 9 to meet with officials of the military-led government.

His visit will follow a similar undertaking last week by Langhorne A. Motley, assistant secretary of state for inter-American affairs, who met with government, opposition and Catholic Church leaders.

Although Sanchez was unavailable yesterday for comment, a State Department official said the Pentagon's message would be delivered to "a different cast of characters."

"Different elements of our government have different lines of communications," he said.

Until recently, the Reagan administration's approach toward the government of Gen. Augusto Pinochet had been described as one of "quiet diplomacy" that human rights activists had criticized as ineffective in promoting a return to democracy.

But a high-level State Department official said yesterday that recent developments in Chile had caused concern in Washington and among "our European friends," leading to a "change in tactics" if not fundamental policy.

The visits of Motley and Sanchez, officials said, coincide with a trip this week by British Undersecretary of Foreign and Commonwealth affairs Sir William Harding.

All three visits are designed, said another State Department official, to "get the government and opposition back into the dialogue mode."

Specifically, he added, the United States wants to get the Pinochet government and the noncommunist opposition to discuss a timetable for a transition to a democratic government.

Sporadic talks between the government and the opposition repeatedly were interrupted over the past two years by street demonstrations and harsh government response and now have collapsed.

Pinochet declared a state of siege on Nov. 6 that was strongly criticized by the United States and other nations.

A department spokesman said recent developments in Chile that have caused concern here are: "the lack of agreement among democratic forces on an agenda and tactics for engaging the government, the government's failure to take the necessary steps to implement its own constitution . . . [and] Communist-backed efforts to increase armed violence and terrorism."

As a result of these factors, the high-ranking official said, the administration has taken a number of actions, including a series of statements and reports critical of the Pinochet government, the cancellation of a visit last year by a U.S. general and the U.S. decision this month to abstain in the vote on a $130 million loan to Chile in the Inter-American Development Bank.

A State Department spokesman said yesterday that Washington is "carefully reviewing" whether or not to vote for another loan of $10 million requested by Chile from the World Bank.

An informed source said the human rights bureau was expected to recommend abstaining in the vote on the loan.

The spokesman yesterday stressed administration concern over the lack of progress toward implentation of a government plan for a gradual transition to democracy.

In a plebiscite held in 1980, whose validity was contested by most political parties, voters approved a constitution providing for an eight-year transition period during which Pinochet and the military junta are to continue to rule the country.

In 1989 Chileans are to vote again, this time on a presidential candidate chosen by the junta.

The spokesman noted that while "53 months have passed" since the constitution was approved, no concrete steps have been taken to draft a timetable for the election of a democratic government.

The department's annual report on human rights in countries around the world noted that in Chile "no progress was made in legalizing political parties, establishing electoral laws and an electoral tribunal or in reestablishing electoral registers."

The human rights report, described by one State Department official as the "toughest yet," also says that under the state of siege there have been many violations of human rights, including mass arrests, censorship and torture.

Administration officials have said it is difficult to influence the Chilean government, in part because it receives no military aid from the United States.

Motley, who briefed reporters Friday after returning from Chile, said it would be counterproductive to urge Pinochet to take specific steps concerning the transition to democracy.

He said he did not believe it was "appropriate" for a "gringo" to try to "muscle" the Chilean leader