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A sweeping new federal law has a seemingly simple goal — curbing offshore tax evasion by Americans through foreign banks, trusts and shell companies. But behind the scenes, foreign banks and financial firms are increasingly finding that complying with the law is a major headache.

Treasury Department officials say they are moving apace in getting the world’s banks on board with the law, the Foreign Account Tax Compliance Act. They say they have reached agreements with some large countries, are working on deals with others and are refining parts of the law, which is set to take effect on June 30, 2014.

But some financial institutions, trade groups, scholars and members of Congress have raised an array of concerns, starting with the cost of creating the complex computer systems needed to track Americans’ accounts.

In addition, tax havens like China, Panama and Russia have yet to sign on. And American banks are unhappy about a Treasury Department pledge to foreign banks, not part of the original law, to require American financial institutions to share data with other countries about foreign investors who have accounts in the United States.

“You can search a long time for comments from the private sector or other governments singing the praises” of the law, said Mark E. Matthews, a tax lawyer at Caplin & Drysdale in Washington, and a former deputy commissioner of the Internal Revenue Service. “It’s all criticism, and that speaks volumes about the challenges.”

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Still, even the critics acknowledge that they cannot stop the law, which aims to become a model for global finance rules, from going into effect. The question is whether all the global financial institutions will comply equally.

The law, known informally as Fatca, effectively makes all foreign banks and foreign financial institutions arms of the I.R.S. by requiring them to disclose data on American clients with accounts containing at least $50,000, or to withhold 30 percent of the dividend, interest and other payments due those clients and to send that money to the I.R.S. The law applies to banks and financial institutions even if their home countries have secrecy laws. Those that do not comply could face significant fines or be locked out of doing business with American clients.

Robert B. Stack, deputy assistant secretary for international tax affairs at Treasury, described the law as a success. “We have worked very closely with financial institutions to come up with a practical, risk-based approach that balances benefits and burdens,” he said in an e-mail. “We think those efforts have paid off.”

Pascal Saint-Amans, director of the Organization for Economic Cooperation and Development’s Center for Tax Policy and Administration, called the regulation “a reality,” adding that “all countries support its underlying policy goals.”

But global banks and investment firms have made their dislike of the law known, though they are reluctant to speak out individually.

Payson Peabody, managing director and tax counsel at the Securities Industry and Financial Markets Association, Wall Street’s main lobbying group, said, “The goals are laudable, but there’s the risk of a train wreck” if some countries and banks do not comply.

Another critic is Georges Ugeux, a dual Belgian-American citizen, a lecturer at Columbia Law School and the founder of Galileo Global Advisors, an international business consulting firm. He described the law as “bullying and selfish.” The United States, he said, “is acting outside its borders as if they were its home.”

Mr. Ugeux also questioned the fact that the law addresses only tax evasion by individuals and not by corporations.

The original legislation did not require that American institutions share with foreign governments any account data on foreign individuals. But in the last two years, as global banks complained about the costs and efforts needed to comply, the Treasury Department took a different tack — signing deals with foreign governments covering their banks, rather than with the banks themselves.

The initial intergovernmental deals, signed in February 2012 with France, Germany, Spain, Italy and Britain, contain the first mention of reciprocal data sharing. A German, say, who lives and works in Britain and who has accounts at American banks may find his data being shared by the I.R.S. with Britain, where he pays taxes.

Internal Revenue Service rules already provide for limited sharing of data, generally covering interest income from bank deposits held by nonresidents. The new intergovernmental agreements declare that the Treasury Department will seek legislative approval to disclose a much broader array of data covering investments, pensions and insurance.

In April, the Florida Bankers Association and the Texas Bankers Association, two trade groups, filed a federal lawsuit seeking to block the law from taking effect, saying that “reciprocity” was spurring nonresident customers to withdraw deposits — $50 million from one unnamed bank.

In May, Senator Rand Paul, a Kentucky Republican, introduced a bill seeking to outlaw parts of the data-sharing provisions. The bill cites privacy concerns.

In July, Representative Bill Posey, a Florida Republican who is on the House Financial Services Committee, wrote to Treasury Secretary Jacob J. Lew to complain that the original legislation did not give the Treasury the authority to seal intergovernmental agreements or share client data.

James George Jatras, the manager of RepealFatca.com and a policy adviser to Senate Republicans from 1985 to 2002, argued that “Treasury is not to eager to volunteer it’s making promises of reciprocity that aren’t enforceable under law.”

Treasury officials counter that they see no legal hurdles to reciprocity. “We believe we have the existing authority,” said one person briefed on the issue who spoke on condition of anonymity.

Asked about sharing data with foreign governments, Mr. Stack said that “countries are not only interested in becoming compliant with Fatca, but they see what they have to gain in learning about their own tax cheats.”

Sabrina Siddiqui, a Treasury spokeswoman, declined to answer questions about China and its money centers, Hong Kong and Macau.

The person briefed on the issue said that the Treasury Department was working through the State Department and the Inter-American Development Bank to try to reach agreements with China and some countries in Latin America and the Caribbean. Other tax havens, including the Netherlands, the British Channel Islands, Israel and Singapore, are in talks about agreements with the United States.

One provision of the new law, the so-called pass-through rule, which governs payments that flow through a bank that is compliant with the new requirements to a noncompliant bank, may be simplified or “may not be necessary as originally contemplated if there is widespread participation” in the rule, said Manal Corwin, Mr. Stack’s predecessor at Treasury and now an international tax partner at KPMG in Washington.