The Senate Finance Committee is pressing the Internal Revenue Service to escalate its policing of offshore accounts for instances of tax evasion. During a hearing yesterday, senators Kerry and Conrad, in particular, criticized the IRS for not doing more to investigate such funds.

“I don’t think you’re taking this seriously,” Mr. Conrad said to the director of the IRS’s Large and Mid-Size Business Division, Frank Ng. “I’ve not had you come one time to me and ask for additional tools to go after this problem.” Mr. Conrad, a Democrat of North Dakota, also said he had been told by tax industry officials that offshore accounts “are growing like a cancer.”

Mr. Ng responded that the IRS has 1,500 employees dedicated to pursuing international tax enforcement, about 2.5% of its overall force.

“It is very difficult to estimate the level of noncompliance,” Mr. Ng said.

The hearing was held in conjunction with the release of a report from the Government Accounting Office, the investigative arm of Congress, on a five-story building in the Cayman Islands. As of March, there were 18,857 entities registered at the Ugland House — a nearly 50% increase in the past four years.

While Americans wholly own only about 5% of the companies, as many as half the companies have billing addresses there. The IRS said about 9,000 of the companies were American, but the picture is complicated: Hedge funds, for example, make up about one-third of the companies at Ugland House, and they are owned by their fund investors, so are not directly owned or controlled in the traditional sense.

A recent investigation by the Senate showed that the use of tax havens has cost America an estimated $100 billion a year in lost revenue, but the IRS said it could not verify the number.

The IRS did report a $345 billion tax gap, which is based on studies conducted as long ago as 2001, as well as audits. “It is the best estimate we have today,” Mr. Ng said, adding: “We cannot say how much of the tax gap is from offshore activity.”

Counsel at the law firm Baker & Hostetler, Jack Blum, told the hearing yesterday that the $100 billion figure is “low and conservative.”

“If you take a look at the analyses, the numbers are enormous,” he said, pointing to the British Virgin Islands, where there are 500,000 entities registered and, unlike the Caymans, the island does not require any reporting. “We have no idea” how many of the companies in BVI are American, he said.

“Ugland House provides an instructive case example of the tremendous challenges facing the U.S. tax system in an increasingly global economy,” the GAO said in its report on the building. While even government-affiliated entities have used some of these services, “and the Cayman Islands government has taken affirmative steps to meet international standards, the ability of U.S. persons to establish entities with relatively little expense in the Cayman Islands and similar jurisdictions facilitates both legal tax minimization and illegal tax evasion,” the report concluded.