Fischer made the remarks at a weekend rally of his Green Party in an attempt to shore up falling public support for the coalition the Greens have with Chancellor Gerhard Schroeder's Social Democrats.
Within the coalition, the Greens were given the responsibility of immigration and in 1999 they loosened the requirements for entry visas. The opposition claims many of the visas issued led to an influx of illegal workers who joined the ever-increasing "black" economy in which Ukrainians in construction and other sectors work for much lower wages. Unemployment is running high in Germany, with more than 5 million out of work in January.
"I did not react with sufficient speed or firmness, or fully enough as the responsible minister," Fischer said. "I've made mistakes."
In recent weeks as details emerged from the Foreign Ministry and the Interior Ministry over how visas were issued without sufficient checks that allowed exploitation by mafias running human trafficking rackets, the issue has snowballed into a political scandal.
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