Story highlights Graco is voluntarily recalling convertible, harnessed booster car seats

But it is not recalling infant car seats, despite the government's request

The problem: The seats' harness can become difficult to unlatch

Those with recalled seats or concerns can get a new buckle, company official says

In one of the biggest such recalls ever, Graco has agreed to recall 3.7 million child car seats manufactured in recent years -- even as it officially contests a government request to recall nearly 1.8 million more -- over a buckling issue.

The National Highway Traffic Safety Administration (NHTSA) announced Tuesday that Graco is voluntarily recalling 11 of 18 model seats that the government agency had asked it to recall.

"NHTSA's investigation will remain open pending its evaluation of the Graco recall and until the agency's consideration of the review of the 7 remaining sea models is completed," the department said.

The 3.7 million seats alone makes this recall the fourth biggest ever for car seats, according to the federal agency. If all the seats the NHTSA was asking to Graco to recall are added in, it would be the biggest.

In a statement, Graco said the voluntary recall affects "harness buckles used on all toddler convertible car seats and harnessed booster seats manufactured from 2009 to July 2013."

"This is not a new issue for us," Graco spokeswoman Ashley Mowrey said, noting that all products sold since July don't have the same problem. "We've been working to help consumers for some time now."

At issue: the red release button in the center of the harness, which the NHTSA said can become difficult to unlatch, or can become stuck. That can make it difficult or impossible to remove the child quickly in an emergency.

Graco explained that it found "that food and dried liquids can make some harness buckles progressively more difficult to open over time or become stuck in the latched position."

The children's products company added that it was offering a "new and improved replacement harness buckle to affected customers at no cost." People can obtain one by calling 800-345-4109 or emailing consumerservices@gracobaby.com.

Until they get such a replacement, Graco believes that parents should continue using their seats for their children.

"This does not, in any way, affect the performance of the car seat or the effectiveness of the buckle to restrain a child," the company said.

The National Highway Traffic Safety Administration, meanwhile, said it "encourages parents and caregivers to consider acquiring an alternative car seat for transporting children until their Graco seat is fixed."

Graco reports no injuries associated with this issue on seats, which sold for between $99 and $400.

According to Graco's website , the recall models include these toddler convertible car seats: the Cozy Cline, Comfort Sport, Classic Ride 50, My Ride 65, My Ride 65 with Safety Surround, My Ride 70, Size4Me 70, My Size 70, Head Wise 70 and Smart Seat.

The harnessed booster seats that are part of the recall are the Nautilus 3-in-1, Nautilus Elite and Argos.

According to the government, the seven models that Graco isn't recalling despite being asked to do so by the National Highway Traffic Safety Administration are all infant seats. Specifically, they are: Snugride, Snugride 30, Snugride 32, Infant Safe Seat-Step 1, Snugride 35, Tuetonia 35, and Snugride Click Connect 40.

Asked about these models, Graco's spokeswoman said that any customers with those or other car seats can still call and get a new harness buckle sent to them for free.

"They are not officially recalled; however, customers experiencing any difficulty with their harness buckle can still get a new one," said Mowrey.