PARIS — Washington won important backing Wednesday for an effort to identify offshore accounts held by Americans, as key European allies agreed to help.

In a joint statement, the United States, France, Germany, Italy, Spain and Britain said they wanted “to intensify their cooperation in combating international tax evasion.” In return, Washington has agreed to “reciprocate in collecting and exchanging” information about U.S. accounts held by residents of those countries.

The agreement concerns the Foreign Account Tax Compliance Act, known as Fatca, which became law as part of a 2010 jobs bill. Fatca is meant to help the U.S. Internal Revenue Service identify hidden accounts and other assets held overseas by Americans, but the law has caused an outcry among foreign financial institutions that fear the cost of compliance as well as what they have said are unrealistically tight implementation deadlines.

There were also concerns that Fatca — which will require that virtually every financial institution in the world report any accounts held by Americans, with a withholding penalty for noncompliance — would put banks into a position of violating national secrecy laws to comply.