Israel's finance ministry has sought help from Switzerland in identifying Israelis it believes have undeclared assets at Union Bancaire Privee (UBP) and Bank Julius Baer, Swiss government documents show.

UBP said on Wednesday it is cooperating fully with authorities, while the Swiss Federal Tax Administration is seeking contact addresses for the account holders and giving them 20-days to provide the data, in advance of deciding whether to assist Israel in its hunt for untaxed money.

Thousands of Israelis have disclosed foreign accounts containing billions of dollars under a government amnesty program in recent years, as they sought to avoid prosecution in a crackdown on unreported capital.

Untaxed money in accounts in Switzerland has been a target for foreign governments for years, with U.S. authorities successfully securing billions of dollars in settlements and penalties from Swiss banks since 2008.

The documents outlining actions involving UBP, published in the Swiss Federal Register on Tuesday, outlines the Israeli Finance Ministry's efforts to identify Israelis with accounts at the private bank between 2014 and 2017 who it alleges have failed to provide sufficient evidence of tax compliance.

A similar document addressing accounts at Bank Julius Baer, which declined to comment, was made public earlier this month.

The Federal Register documents show that in both instances the private banks asked account holders with an Israeli address to document their tax compliance. Those who did not show sufficient evidence were told their business relationship with the bank would be terminated.

"UBP is one of the banks that have received a request from the Swiss tax authorities acting on a demand from the Israeli tax authorities, and has taken the necessary measures to abide by this request," the Geneva-based private bank said.

"UBP has actively promoted Israel's tax amnesty programs," the bank said, adding it had also done so for similar disclosure programs in Mexico, Brazil and Turkey.

'FISHING EXPEDITION'

Switzerland has forged agreements with numerous other nations to automatically exchange bank information amid scrutiny of its role in managing offshore wealth.

Swiss banks cannot deliver account information directly to other governments, only to the Swiss tax authorities, who may then transfer it to a requesting tax agency.