Metro officials halted the use yesterday of all buses manufactured by Neoplan U.S.A. Corp., saying that new tests had detected possibly dangerous cracks in 44 additional vehicles.

Since last week, the transit authority has reported finding hairline and other cracks near the front axles of 75 of the 76 buses it purchased from the West German-affiliated company for $12.5 million in 1983. One other Neoplan bus was destroyed last December in a fire attributed to faulty electrical circuits.

"We have taken all Neoplan buses out of service because the cracks pose a potential safety hazard for our operators and passengers," Metro Deputy General Manager Theodore G. Weigle Jr. said in a statement. He described the action as "a precautionary measure."

Metro officials have warned that the cracks, found in steel plates and tubes and in welds in a key section of the buses' supporting frames, might result in a loss of control over steering. Metro has about 1,600 buses in its fleet.

The Colorado-based company, which is affiliated with a West German bus manufacturer, Gottlob Auwaerter GMBH & Co., issued a statement saying that Metro officials had "certainly overreacted" by removing the Neoplan buses from service.

"It remains Neoplan's determination that, at this point, no safety issue is involved," the statement added. "Only hairline cracks have been found, and any more severe characterizations of the condition are totally exaggerated."

Metro officials said that the new cracks were found Monday night when the buses were tested with a special reddish dye designed to reveal flaws that may not be evident in routine inspections. The dye accentuates tiny cracks by making them appear darker.

Officials said the tests showed evidence of cracks in every Neoplan bus that was still in service. All were sidelined immediately, they said. The Neoplan buses were assigned mainly to routes serving Southeast Washington, partly in an attempt to improve service in Anacostia and other low-income areas.

A Metro spokesman said that the Neoplan buses now have been replaced primarily by recently renovated vehicles in an effort to bolster service in Southeast. The refurbished buses were taken from other garages serving Metrobus routes elsewhere in the area, the spokesman said. These garages have been given older buses from Metro's reserve fleet.

In its statement yesterday, Neoplan said it had "already diagnosed the causes for the cracks" in the Metrobuses. A "technical solution has been developed, which can be implemented on the buses in a short time period," the statement said.

Asked for comment last night, a Metro spokeswoman said, "We hope that their assessment is accurate."

Neoplan officials were not available to elaborate on the proposed remedy. Metro officials have said they previously rejected a Neoplan proposal for repairing the cracks, terming it inadequate. Weigle said that officials were "actively engaged in discussions with Neoplan on possible remedies."

Neoplan and other buses have posed repeated problems in recent years for Metro and other transit systems throughout the United States.

Metro's board of directors voted to buy the 76 Neoplan buses in June 1983 over protests from one board member, Fairfax County Supervisor Joseph Alexander, who criticized the move by citing evidence of defects in Neoplan vehicles bought by other transit systems. Neoplan had submitted the lowest bid.

Last year, hairline cracks were found in welds above the buses' rear axles. Two buses were damaged in fires blamed on improper electrical wiring.

In December, the National Highway Traffic Safety Administration, citing Metro's problems, urged Neoplan to recall the nearly 2,000 buses it has sold nationwide.

Other transit systems also have reported defects in Neoplan buses. In Los Angeles, cracks were found in 128 Neoplan vehicles last September and new cracks were found in 92 more buses in the past two months, a spokesman said.

A year ago, the New York City Transit Authority permanently withdrew from service 850 buses made by another manufacturer, Grumman Flxible Corp. The authority cited cracks and other defects, which it described as safety issues. The city is seeking to sell them.