U.S. antitrust regulators approved Google Inc.'s $3.1 billion purchase of DoubleClick Inc. Thursday, removing a key obstacle to a formidable combination in the burgeoning online advertising sector.

The transaction still faces substantial antitrust scrutiny from European regulators and cannot be completed without their approval. The European Commission has set a deadline of April 2 to finish its review.

The Federal Trade Commission appeared to accept many of Google's arguments that its online ad sales business doesn't compete with DoubleClick's ad-serving tools, saying its analysis "showed that the companies are not direct competitors in any relevant antitrust market."

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