(Reuters) — U.S. antitrust enforcers are dealing with a "tsunami" of high-value, complicated mergers that have stretched their resources, the heads of the two U.S. competition regulators told lawmakers on Wednesday.

Bill Baer, head of the U.S. Justice Department's Antitrust Division, and Edith Ramirez, chairwoman of the Federal Trade Commission, told a Senate panel that 2015 was exceptionally busy with a large number of complex deals.

"There was mention of a merger wave. We kind of look at it as a tsunami," Mr. Baer told the Senate Judiciary Committee's antitrust subcommittee. He attributed some of the volume to investors who jumped in once the economy improved.

"We are incredibly busy. We are asking for additional resources," said Ms. Ramirez, who did not elaborate. Mr. Baer has asked for an additional $16 million for a total of $180 million in funding for the antitrust division.

But while volume was higher, the percentage of problematic mergers was unchanged, said Mr. Baer. The FTC sued to block six deals in the past year while the Justice Department opposed 11 in the 2015 and 2016 fiscal years.

Lawmakers expressed concerns about two pending health insurance mergers and their possible impacts on the cost of health care and drugs. Anthem Inc. announced its proposed $47 billion acquisition of Cigna Corp. last July, the same month that Aetna Inc. said it planned to buy Humana Inc. for $37 billion.

The Justice Department is currently reviewing both deals. Mr. Baer said they would be assessed both locally and nationally, including the cost to employers and impact on markets. "These are transformational mergers. We are going from five national (insurers) to three."

Under questioning from Sen. Charles Grassley, R-Iowa, Mr. Baer said the department would consider concerns that patients would be unable to see their preferred doctor if a deal was approved.

Ms. Ramirez indicated that the FTC was investigating several strategies used by brand-name drug companies to defend their medicines, including moves to block generic companies from obtaining needed drug samples.

Sen. Amy Klobuchar, D-Minn., the top Democrat on the panel, pushed for the FTC to be aggressive, saying, "We need increased enforcement actions to stop anticompetitive actions in the pharmaceutical industry."