Metro officials yesterday removed 23 recently purchased buses from service, warning that "severe cracking" in the vehicles' undercarriages posed "a potential safety problem."

The cracks, found in buses manufactured by Neoplan U.S.A. Corp., a West German-affiliated company, could result in loss of control over steering, officials said. The damage was detected in metal components connecting the front axles to the main frames supporting the buses.

"I don't see how we could control the steering if that axle assembly fell out," said Shirley A. DeLibero, Metro's assistant general manager for bus service. "You'd lose your whole wheel suspension."

The Neoplan buses have repeatedly presented problems and have undergone several major repairs in the past year. Hairline cracks were previously found in welds above the rear axles.

Later, two bus fires occurred because of what was described as faulty electrical circuits.

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

Yesterday Neoplan, a Colorado-based company affiliated with the West German bus manufacturer Gottlob Auwaerter GMBH & Co., issued a statement saying, "We do not think that, at this point, a safety issue is involved."

Nevertheless, the statement added that Neoplan "certainly will not argue" with Metro's decision to withdraw the buses from service for repairs. The statement said it expected all to be returned to service "after a couple of hours' repair work."

Yesterday marked the first time Metro officials had characterized defects in the Neoplan buses as a safety issue, and they expressed uncertainty about prospects for repairs.

Officials said they would consider withdrawing the remaining 52 Neoplan buses. One other bus was destroyed in a fire.

"If we continued to let them run, we don't know what the hell would happen," DeLibero said. "We don't want to take the chance."

The Neoplan buses had been assigned mainly to routes serving Southeast Washington, partly in an attempt to improve bus service in Anacostia and other low-income areas.

The new vehicles were expected to be less prone to breakdowns than Metro's older buses.

However, the buses' recurrent troubles have thrown Metro's plans awry. Other older buses were taken from Metro's reserve fleet to replace the Neoplan vehicles and avert any interruption in service, officials said.

The first evidence of cracks near the front axles of the Neoplan buses was detected Jan. 18, when a 3-inch hairline split was found in a metal tube in the undercarriage of one bus. Officials said a check of other buses found no similar flaws at that time.

However, Metro inspectors spotted cracks Wednesday night in the same section of another bus, which is known as the front axle subframe assembly.

An immediate inspection turned up cracks in seven more buses. When the remaining Neoplan buses were called in for inspections, officials said the total rose to 23.

The cracks were found in metal plates, metal tubes and welds connecting these components.

In one instance, a plate attached to an axle assembly became separated from the main frame of a bus. No accidents have been reported because of the cracks. Said one Metro official, "We were just plain lucky."

Controversy has surrounded the Neoplan buses since the transit authority's board of directors voted to buy 76 vehicles for $12.5 million in June 1983. Fairfax County Supervisor Joseph Alexander, a Metro board member, sharply criticized the move, contending that Neoplan buses often were found defective.

In another development yesterday, Metro officials recommended a plan to improve the subway system's trouble-plagued Farecard system.

The proposal, which received preliminary approval from a Metro board committee, would require the agency to negotiate a contract with Cubic Corp., the fare-collecting equipment's manufacturer, to overhaul the balky machines at an estimated cost of $22.7 million.

A Metro committee also approved a compromise plan to equip half the buses purchased by Metro this year with special lifts for handicapped riders.