The US National Highway Traffic Safety Administration (NHTSA) has intensified an investigation into over two million General Motors cars after 600 complaints of engine problems that could lead to a fire, Reuters reported.

According to Reuters, the NHTSA also said on Wednesday it had opened three separate investigations into defects with popular Toyota, Honda and Nissan family saloons.

Reuters said the NHTSA GM investigation covered about 2.3 million Buick, Chevrolet, Pontiac and Oldsmobile models built between 1996 and 2000 with 3.8 liter V6 engines. According to Reuters, NHTSA said 604 consumers had complained either to GM or the agency that an engine backfire had fractured the intake manifold, with 44 reports to NHTSA of fires caused by fuel leaking from the broken manifolds.

Reuters said GM had recalled 276,000 V6-powered cars in 1996 for a similar problem, and NHTSA said the car maker had made three design changes to models built since then aimed at dealing with backfiring. NHTSA also said the number of complaints had been rising over the past six years, Reuters added.

According to Reuters, the NHTSA said GM was contending the problem was not a safety concern because it was rare, there were few reports of injuries and the problem happens during start-up, not while the vehicle is in motion.

Reuters said NHTSA's move made the investigation an engineering analysis, one step away from a recall and added that NHTSA rarely orders car makers to recall vehicles; either car makers issue their own recalls, or the investigations find no safety defect.

According to Reuters, the largest of three NHTSA investigations involving Asian-designed midsize cars was triggered by complaints of fires in Toyota Camry sedans, one of the top-selling cars in the United States, sold between model year 1997 and 2002.

NHTSA told Reuters it had received 35 complaints, including reports of two crashes and one injury, stemming from fires in the engine compartment of Camrys. The agency did not estimate how many Camrys the investigation covered, but Toyota regularly sells more than 400,000 a year in the United States, Reuters said.

The news agency said a similar problem started an investigation into 2002 Nissan Altima sedans with four-cylinder engines. NHTSA told Reuters it had three complaints of engine compartment fires in Altimas, and that all three vehicles were written-off.

Reuters said 10 complaints of seat frame bracket failures in 1999-2002 Honda Accords triggered another NHTSA investigation. The agency said the complaints allege the bracket can come loose, allowing the seat to fall backward without warning, Reuters added.

According to Reuters, all the investigations are classified as "preliminary evaluations," where NHTSA asks for complaint data from car makers to determine whether a trend exists and whether a problem warrants a closer look.

Reuters added that the agency also revealed a number of other investigations on Wednesday, including an upgraded look at 262,000 Jeep Grand Cherokee sport utility vehicles from the 1997 model year to more closely closer examine complaints that the Grand Cherokee's hood latches can fail, allowing the hood to fly open while the vehicle is in motion; a preliminary investigation into 265,000 Honda-made Acura 3.2 TL and 3.2 CL cars built between model years 2001 and 2003 over 35 complaints of transmissions that can fail without warning while the cars are moving; investigating 272,000 Ford Taurus and Mercury Sable cars from the 1999 model year, after seven complaints to NHTSA that a front suspension spring can break, and in some cases puncture a tyre.