JACKSON, Miss. — Nissan officials are on the defensive after four of five vehicles produced at a key plant in the automaker's expansion plans topped the annual Consumer Reports list of unreliable vehicles.

Nissan officials acknowledge there were some initial problems with the vehicles produced at its sprawling plant in Canton, Miss., which opened in 2003, but they point to other surveys showing the reliability of vehicles made there is improving.

Among the Canton-made vehicles that landed in the "unreliable" category of Consumer Reports' study — which was based on surveys of 1 million consumers — were Nissan's Armada sport utility vehicle, Quest minivan and Titan pickup, and the Infiniti QX56 SUV.

The Altima sedan — also built at Nissan's old Smyrna, Tenn., plant — was the only Canton-made vehicle that was not near the bottom of the list.

"It's difficult to actually put a reason on why they are so unreliable," said David Champion, director of automobile testing for Consumer Reports, a publication of the nonprofit Consumers Union. "But it is a little bit worrying when the worst plant in the U.S. is actually a Nissan import plant."

Champion said it is the first time in his eight years on the job that a single Japanese auto plant had such a poor showing.

Carlos Ghosn, Nissan's CEO, told The Clarion-Ledger (Miss.) newspaper during a recent tour of the plant that Nissan officials "were disappointed when the first quality numbers came out."

But he added that "we've taken the challenge to bring up our numbers."

The Consumer Reports study does not reflect the improvements made since the 4,200-employee plant opened, said Nissan spokeswoman Vicki Smith. She noted another study released last May from Strategic Vision that showed Nissan to be the best full-line corporation in their quality index.

A spokesman for Strategic Vision, a San Diego-based automotive research and consulting firm hired by Nissan, said most of the 90,000 owners they interviewed were happy with their vehicles.

Consumer Reports found that the Titan pickup and the Armada SUV had an array of problems — from faulty fuel systems to defective electric windows — but those same vehicles came out on top of Strategic Vision's quality report.

The Titan also won "most satisfying" overall vehicle in September in the AutoPacific Vehicle Satisfaction Awards, which involved interviews with about 35,000 Nissan owners. The Armada was the Tustin, Calif.-based automotive consultant's best SUV, and the Quest tied for the top spot in the minivan category.

"They've paid a lot of attention to what the customer wants and they've come close to giving it to them," said AutoPacific President George Peterson. "There's just always a learning curve at a new plant with new products."

Chance Parker, an executive director with J.D. Power and Associates, which also rates vehicle dependability, said the automobiles built in Canton had only about half as many problems this year as the previous year. Models such as the Quest minivan was J.D. Power's most improved vehicle in 2005, but Parker said it is still below average in reliability.

In 2004, J.D. Power found that the owners of some vehicles produced at the plant complained of rattles, leaks, vibrations and paint problems. Nissan brought in a team of engineers from Japan to address those problems, but some consumers still have complaints.

So, what was wrong with the Canton cars? Parker said there is "no smoking gun. The cars had problems in a number of areas.

"They were dealing with an all-new plant, all-new workers and all-new vehicle designs. In the best case, an all-new vehicle design is a challenge," Parker said. "So when you add all those things together, Nissan faced kind of a perfect storm as far as quality is concerned."

Parker expects the reliability of automobiles produced in Canton to continue to improve.

But Consumer Reports' Champion is not so optimistic.

To put the problems in perspective, Champion said the most reliable car Consumer Reports found was the Toyota Prius, and only 4 percent of that car's owners reported problems with the vehicle. "If you look at the worst-reliable vehicle, which was the Infiniti QX56, 40 percent of those owners reported having problems with their vehicles," he said.