A Swiss lawyer accused of helping US clients conceal money in offshore accounts has pleaded guilty to conspiracy to commit tax fraud in federal court in New York, media reports said.

Edgar Paltzer, a dual US-Swiss citizen, admitted to opening bank accounts in the Alpine state in the name of entities he formed for US citizens, knowing they intended to evade taxes.

As part of his plea, entered last Friday, Paltzer agreed to forfeit any fees he earned and cooperate with the US government. "His co-operation is complete and without any limitation," Thomas Ostrander, Paltzer's lawyer, said after the hearing, Reuters reported.

Paltzer, a former partner of Swiss law firm Niederer Kraft & Frey who is licensed to practise in New York, was first charged in April on one count of conspiracy alongside Stefan Buck, then head of private banking at Bank Frey & Co. That indictment accused them of conspiring with US taxpayers from 2000 through at least 2012 to help them hide their Swiss accounts from the Internal Revenue Service.

Prosecutors say the small bank gained US clients as the Justice Department pursued other banks.

US authorities say that by the third quarter of 2012, about 44 per cent of the bank's assets under management were for US taxpayers – that number surged between February 2009 and February 2012 by 300 per cent. This period coincides with crackdowns by US authorities on Swiss banks UBS and Wegelin.