DETROIT — Thousands of Chrysler LLC autoworkers walked off the job today after the automaker and the United Auto Workers union failed to reach a tentative contract agreement before a union-imposed deadline.

It is the first UAW strike against Chrysler since 1997, when one plant was shut down for a month, and the first strike against Chrysler during contract talks since 1985. Negotiators stopped talking after the strike began, according to a person briefed on the talks who requested anonymity because the talks are private.

The UAW apparently is not striking at five plants that Chrysler already had idled this week because of sagging sales of some models, according to another person familiar with the walkout who asked not to be identified because the situation is in flux.

A forklift driver at the Sterling Heights assembly plant in suburban Detroit, Brett Ward, said he thinks a strike is justified, but he hopes the union can get a better deal than the one it reached with General Motors Corp.

“Hopefully with a strike we’ll get some better gains and get a better contract in front of us,” he said.

The UAW, which must reach new four-year agreements with all three Detroit automakers, struck GM for two days before tentatively settling with the automaker on September 26. The union hasn’t yet agreed with Ford Motor Co.

Chrysler has 24 American manufacturing facilities, including 10 assembly plants. The automaker had already planned to idle five assembly plants and some parts making factories for short stretches during the next two weeks in an effort to adjust its inventory to a slowing American automotive market.

Workers didn’t strike the Warren Truck assembly plants in Warren, Mich.; Newark, Del., assembly; Jefferson North assembly in Detroit; Belvidere assembly in Belvidere, Ill., and the Conner Avenue assembly plant in Detroit.

A short strike likely will have little effect on the automaker, which had a 71-day supply of cars and trucks on dealer lots at the end of August, according to Ward’s AutoInfoBank. A walkout longer than a month would start to cut into sales, a chief economist with the National Automobile Dealers Association, Paul Taylor, said.

Talks between the UAW and Chrysler began in July but accelerated last weekend. The union set the 11 a.m. deadline to settle or to strike. The UAW represents about 45,000 workers at Chrysler’s American manufacturing facilities, making it the smallest of the Detroit automakers.

Chrysler was a wild card in this year’s negotiations because it was bought by the private equity firm Cerberus Capital Management LP shortly after the talks began. DaimlerChrysler AG, which is now called Daimler AG, sold a controlling stake in the 82-year-old Chrysler to Cerberus in August. The firm has since hired Bob Nardelli, formerly head of The Home Depot Inc., to be Chrysler’s chairman and chief executive. Chrysler’s vice chairman and president, Tom LaSorda, who led Chrysler before Nardelli was hired, is representing the company in the talks.

Many industry analysts believe Cerberus will fix the money-losing Chrysler quickly, return it to profitability, and sell it for a huge profit, perhaps to a foreign auto company that wants a stronger American presence. It was unclear how Cerberus’s plans for the company would factor in the talks.

The bargaining appeared to hinge on the UAW granting the same health care cost concessions to Chrysler as it did to GM and Ford in 2005, and on how much Chrysler would pay into a company-funded, UAW-run trust that would take on its roughly $18 billion worth of retiree health care costs. GM has already agreed to form such a trust.

Also at issue was the union’s desire for job security pledges at American factories and Chrysler’s wish to contract out parts transportation now done by higher-wage union members, according to one of the people briefed on the talks, who asked not to be named because the talks are private.

The union normally settles with one American automaker and then uses that deal as a pattern for an agreement with the other two. But several industry analysts have said that Chrysler and Ford have different needs and therefore need different contracts.

Agreements must be ratified by UAW members to go into effect. GM’s 74,000 UAW members have been voting on their agreement for the last week and totals were expected today.