DETROIT (AP) — The nation's auto safety agency is telling Chrysler to speed up a recall of 1.5 million older Jeeps with gas tanks that can rupture in a rear collision.

The National Highway Traffic Safety Administration sent a letter to Chrysler on Wednesday saying that a "woeful" 3 percent of the Jeeps have been fixed, more than a year after the recall began. The agency urged owners to take the Jeeps to dealers for repairs.

It's the latest skirmish in a 4½-year feud between Chrysler and the agency over the recall, which Chrysler initially resisted, saying the Jeeps performed no worse than similar vehicles.

The two sides were headed for a public hearing and court battle until former Transportation Secretary Ray LaHood intervened, and they compromised last year. The compromise still left some vehicles with the same tanks outside the recall and without repairs.

The recall covers 2002-2007 Jeep Liberty and 1993-1998 Jeep Grand Cherokee SUVs, which have gas tanks behind the rear axles that can rupture in rear collisions and catch fire. The remedy is to install a trailer hitch to protect the plastic tanks in low-speed collisions. At least 51 people have died in crashes due to the problem.

The letter, from NHTSA Deputy Administrator David Friedman, says Jeep owners have complained about being turned away by dealers for lack of parts, and in some cases, they're being told the Jeeps are safe to drive without the repair.

"Chrysler must re-examine and accelerate its efforts to repair the recalled vehicles and proactively reach out to their owners," Friedman wrote. He also told the company to confirm in writing that its dealers are giving owners accurate information and doing the repairs quickly.

Dealers also have told owners that they can't install the hitches because the Jeeps have rusty frames, Friedman wrote. He also asked Chrysler for a plan to deal with those vehicles.

Chrysler said it is redoubling efforts to get parts to dealers and will ramp up outreach to customers to get them to make service appointments.

The company maintains that the Jeeps are among the safest of similar vehicles from their era, and that it did the recall because it "recognizes and understands that this issue has raised concerns for customers."