US authorities Monday extended by some three weeks a tax amnesty deadline for holders of undisclosed offshore bank accounts, citing an influx of people facing 'logistical' issues coming forward. The Internal Revenue Service said the deadline, which had been set for Wednesday, was extended to October 15, but indicated there would be no further postponements. 'IRS officials decided to extend this deadline after receiving repeated requests from tax practitioners and attorneys around the country following an influx of taxpayer requests,' the tax agency said in a statement. 'By extending the deadline for a short period of time, the IRS is providing relief for those taxpayers who had intended to come forward prior to the deadline, but faced logistical and administrative challenges in meeting it.' The IRS reiterated that persons 'who do not voluntarily disclose their hidden accounts by the new deadline face much harsher civil penalties, where applicable, and possible criminal prosecution.' The announcement came as Americans with offshore bank accounts were scrambling ahead of Wednesday's deadline to square up back taxes and avoid a likely wave of prosecutions following a disclosure agreement with Swiss bank UBS. US authorities are moving to crack down on overseas accounts for tax evasion, as highlighted by the settlement with UBS last month to reveal the identities of some 4,450 of its American clients. The landmark settlement challenged Switzerland's longstanding banking secrecy tradition and was expected to open the door to more prosecution efforts for tax evasion. The fate of the 4,450 people on the UBS list remained murky, legal experts said. Those UBS clients may ask for amnesty under the program but might not qualify if the government already has their names and has begun an investigation, say attorneys. Legal experts say the threat of prosecution may prompt a mad rush of Americans with accounts at UBS and other Swiss banks or in other offshore havens to come forward. A series of US agreements with other countries will make it easier for authorities to identify and prosecute US taxpayers hiding money and assets overseas. Attorneys say US information-sharing agreements have been signed with Monaco, Gibraltar, Lichtenstein, Jersey, Isle of Man, Guernsey, Aruba, and the Bahamas. The US is not the only country cracking down on offshore tax evasion. France said persons with hidden accounts in Switzerland would be given until December 31 to pay any overdue tax -- after which they would face a full tax audit. The Group of 20 key developed and developing countries in April pledged to block the 'black holes' in the financial system, arguing that the ease with which some market players hide their profits offshore contributes to instability. 