FRANKFURT — Volkswagen said on Thursday that more engines than previously disclosed might be in violation of European pollution rules, but that the number was relatively small.

The engines — installed in models like the Golf, Passat and Jetta early in 2012 — may have software designed to provide an artificially low emissions reading when a car is undergoing official testing. The company declined to say how many vehicles might be affected, but said they were sold only for a few months in 2012. Later versions of the engine are in compliance with European rules, Volkswagen said.

Even if the cars are found to violate European rules, it would not expand the number of cars considered to be in violation in the United States. Europe’s pollution regulations are less stringent than those in the United States, and the vehicles are already considered in violation by the Environmental Protection Agency, that organization said on Thursday.

Volkswagen is in the throes of crisis after the E.P.A. said on Sept. 18 that Volkswagen cars sold since 2009 contained software designed to trick pollution-testing devices by detecting when a vehicle was undergoing emission checks and turning on pollution controls. When not undergoing testing, the cars’ emission controls would be switched off. As a result, vehicles could emit up to 40 times the allowed amount of nitrogen oxides, a component of smog that is linked to numerous lung ailments.