Treasury Secretary Jack Lew issued a strong defense of the administration's efforts on financial reform Wednesday, declaring that "Wall Street reform is working" and claiming that the phenomenon of too-big-to-fail has been ended.

Speaking at a Brookings Institution event on the five-year anniversary of the Dodd-Frank financial reform law, Lew said that the law "ended 'too big to fail' as a matter of law."

Critics on both the Left and Right have challenged the law's claim to prevent future crises or taxpayer bailouts of financial firms. Republicans have also said that it strangles growth and have advanced legislation to roll it back.

Lew's defense comes as House Republicans are preparing a series of hearings and speeches sure to be highly critical of the law.

In Wednesday's speech, Lew cited new capital rules that have caused banks to raise $600 billion in equity as evidence the law is working. He also pointed to new rules on derivatives, speculative trading by banks, and hedge fund activities.

"We have made a world of progress, and I think that we have done the things that we need to do to protect our financial system," Lew said during a question-and-answer session following his speech. He added that he hoped the law would not be given the "ultimate test" of a financial crisis.

"There is more to do," Lew acknowledged, "but that will always be the case because the financial system is always evolving."

In his comments, the Treasury secretary also downplayed the possible ramifications of financial turmoil in Greece and China to the U.S. economy. "Right now, I don't see any immediate threat to the continued growth of the U.S. economy," Lew said.