Democratic presidential candidate Bernie Sanders has tied Hillary Clinton to the Panama Papers, claiming that her past support of the Panama free trade agreement makes her unqualified to be US president.

The White House contender made the accusation as the names of wealthy Americans appearing in the Panama Papers were revealed and US government officials said they would soon be issuing rules to force US shell companies to reveal the identifies of their beneficial owners.

Mr Sanders has claimed Ms Clinton’s support as Barack Obama’s first secretary of state for the trade agreement he signed with Panama in 2011 linked her to the pact that has been exploited by wealthy individuals and corporations to avoid taxes through the Central American nation.

The Vermont senator escalated his attacks on his rival using this week’s leak of millions of documents that showed the use of a law firm in Panama to conceal wealth in secretive shell companies and offshore tax shelters.

Ms Clinton’s support of the Panama trade deal, which Mr Sanders challenged in the US senate, was one reason why she should not be the party’s nominee for president in November’s White House election, he said.

"I don't think you are qualified if you supported the Panama free trade agreement, something I very strongly opposed, which has made it easier for wealthy people and corporations all over the world to avoid paying taxes owed to their countries," the Democratic candidate told almost 13,000 supporters at a campaign rally in Philadelphia on Wednesday.

Critics of the US-Panama agreement have argued that the pact helped increase tax secrecy in the Latin American country.

Mr Sanders has seized on the issue since the Panama Papers first started being published on Sunday, pointing out that he predicted five years ago that the “disastrous trade deal” would make it easier for big companies to avoid taxes by sheltering money offshore.

Media revelations

A speech the democratic socialist delivered on the Senate floor in 2011, strongly opposing the Panama agreement over that issue, went viral online this week after media revelations appeared around the world showing politicians and business people using offshore structures to hide money.

The information was sourced from more than 11 million internal documents at the Mossack Fonseca law firm in Panama that were obtained by German newspaper Süddeutsche Zeitung and shared with the Washington-based International Consortium of Investigative Journalists and 107 media outlets in 76 countries, including The Irish Times.

On Tuesday, Mr Sanders promised to terminate the Panama free trade agreement within the first six months of his presidency if elected.

Trade deals have become a lightning-rod issue in the US election, with Democratic and Republican candidates both blaming the agreements for the loss of American jobs to cheaper overseas countries.

Exit polls in the election contests so far have shown the economy and trade is a major concern for primary voters in both parties.

More than half of voters in Wisconsin’s Republican primary on Tuesday, the most recent contest in the election, said trade with other countries cost US jobs. Democratic voters were divided on the issue.

Mr Sanders and Republican frontrunner Donald Trump have criticised the free trade policies of Mr Obama, while Ms Clinton has become a recent anti-trade convert during the race for the White House, despite her support for the policies while serving in the president's cabinet.

The issue has electrified grassroots Republicans and Democrats alike, helping to drive the popularity of Mr Sanders’s and Mr Trump’s campaigns and putting international agreements high on the political agenda.

Responding to the Panama leaks at a campaign stop in Philadelphia on Wednesday, Ms Clinton vowed to shut down a system that allowed the wealthy to hide money overseas. She made no reference to her support for the Panama agreement during her critique.

"Some of you may have just heard about these disclosures about outrageous tax havens and loopholes super-rich people across the world are exploiting in Panama and elsewhere," she said at the annual convention of the AFL-CIO trade union in Philadelphia.

‘Going after scams’

“We are going after all these scams and make sure everyone pays their fair share here in America.”

Mr Obama has joined a chorus of voices criticising the practices in Panama, though has not linked the issue to the trade deal he signed.

The president told reporters on Tuesday that “this big dump of data coming out of Panama” showed that “tax avoidance is a big global problem”.

“A lot of it is legal but that’s exactly the problem,” he said.

“It’s not that they’re breaking the laws, it’s that the laws are so poorly designed that they allow people if they have got enough lawyers and enough accountants to wiggle out of responsibilities that ordinary citizens are having to abide by.”

Ms Clinton echoed the president’s comments in Philadelphia, saying it was “scandalous” how many of the practices were “actually legal”.

In response to the Panama Papers, the US Treasury Department has said it plans to issue a long-delayed rule shortly, forcing banks to seek the identities of people behind the accounts of shell companies.

Among the high-profile US individuals to appear in the offshore papers are music and film mogul David Geffen, former Hollywood child star and heiress to the Hyatt hotels fortune Liesel Pritzker Simmons, and her husband Ian Simmons, himself an heir to a construction fortune.

Ms Simmons is a relative of US commerce secretary Penny Pritzker.

Another American name to feature in the Mossack Fonseca files is John Thompson Dorrance IV, a descendent of the family that created the Campbell Soup company.

Irish resident

Mr Dorrance's father, John Dorrance III, is an Irish resident and one of the richest people in Ireland.

A US corporate services agency based in Wyoming with links to Mossack Fonseca was audited by the Wyoming secretary of state this week following the leak of the information in Panama.

The company was found to have failed to maintain the required statutory information for performing the duties of a registered agent under Wyoming law.

Mossack Fonseca has touted the benefits of setting up US companies in Wyoming to potential customers in its marketing information.

“Ownership is confidential and under state law may only be obtained by court order,” said the firm.

“Managers and members can either be corporate entities or natural persons,” said the firm, meaning a shell company can be formed in Wyoming without any individual being named in the company’s records.