A Mexicana Airlines Boeing 727-200 with 166 passengers and crew aboard crashed in the mountains about 90 miles northwest of here shortly after takeoff this morning, and there were no survivors, airline and government officials said.

At least three U.S. citizens were among the dead, a Mexicana spokesman said tonight. He identified them as Chris and Tracy Pittner of Buffalo, N.Y., and Susan Dick, a resident of Mexico City, whose Mexican husband and child also died in the crash, the Mexicana official said.

The airline spokesman said it was the worst disaster in Mexican aviation history. A statement issued by the company said that the accident was Mexicana's first in 17 years. On June 4, 1969, 79 persons died when a Mexican Boeing 727 rammed into a mountain near Monterrey in northeastern Mexico.

Carrying 158 passengers and eight crew, the Mexicana jet, flight 940, left Mexico City at 8:40 a.m. en route to the coastal resort of Puerto Vallarta, the first of its two scheduled domestic stops.

At 9:04 a.m., in the last recorded contact with the jet, pilot Carlos Guardarrama Sistos radioed to Mexico City flight controllers that his plane was losing altitude and was having pressurization problems, Mexicana said in a brief press release.

Mexican Air Force helicopters spotted the burning wreckage of the plane soon afterward in a remote mountain area about 90 miles northwest of here, Mexicana spokesman Jose Henonen said.

According to Mexican press interviews with witnesses, the plane burst into flames before crashing. The explosion reportedly split the fuselage in half and scattered pieces of the plane over a square mile of wooded mountainside. "There definitely were no survivors," Henonen said.

The cause of the crash is still undetermined, and rescue workers continue to search for the black flight-recording box, Mexicana officials said tonight.

A passenger list for the flight showed 155 passengers, including 17 with non-Hispanic surnames. Three infants also were aboard the flight, Mexican officials said.

[Airline spokesman Eduardo Kuri said only two passengers, the son and daughter of the pilot, were bound for Los Angeles, the planes ultimate destination. The rest, including nine French citizens, two Canadians and a still undetermined number of other foreigners, were heading to Puerto Vallarta and Mazatlan, United Press International reported.]

Because the first leg of the flight was a domestic route, the manifest did not give passengers' nationalities, Henonen said.

U.S. consular officials were dispatched to the crash site this morning, U.S. Embassy spokesman Vince Hovanec said.

The crash site is located "in a mountainous, difficult area" near San Miguel el Alto, a small rural settlement near the Michoacan town of Maravatio, Henonen said. Mexican Red Cross officials in Maravatio said the site is inaccessible by vehicles and difficult to reach on foot.

The airport at Morelia, the capital of Michoacan state about 45 miles west of the crash site, made arrangements to receive the bodies.

President Miguel de la Madrid today ordered a "full investigation" into the cause of the crash, Ricardo Ampudia, a presidential spokesman, said.

The Boeing 727 has the second-best safety record of all types of commercial aircraft, according to aviation statistics.

In Seattle, Boeing spokesman Dick Schleh said the plane that crashed today was delivered to Mexicana Airlines in May 1981 as one of 39 727s the airline has purchased from the company, UPI reported. Mexicana Airlines, founded 65 years ago, operates 45 jets.

Leaders of Mexicana's union, who asked not to be identified, charged in conversations with reporters today that the airline's management ignored mechanics' complaints about lax aircraft maintenance.

In a private meeting two weeks ago with Mexican chairman Manuel Sosa de la Vesa, union representatives presented accusations about the alleged deterioration of the company's maintenance standards, Henonen confirmed today.

The Mexican government took over majority ownership of Mexicana in an emergency bail-out in 1982 following a period of strikes and economic setbacks for the company. Mexican officials have privately confirmed reports that the airline will soon be sold to private investors.

The crash occurred as Mexicans were returning home from Easter holidays, Mexico's busiest week for air traffic. Mexican airlines said they were operating as many as 50 additional daily flights last week to handle vacation traffic to the country's beach resorts.

At Mexico City's crowded international airport today, stunned Mexicana emmployes wept and listened to transistor radios for details of the accident.