WASHINGTON/TOKYO - The US Justice Department has opened a criminal investigation involving the leaked Panama Papers on offshore shell companies, according to an official letter to the group that brought the records to light.

The letter from the office of Manhattan US Attorney Preet Bharara, obtained by AFP Wednesday, asks to discuss the investigation with the International Consortium of Investigative Journalists, which worked with media outlets to reveal the documents.

The letter, dated April 3 and obtained from the ICIJ, said Bharara's office "has opened a criminal investigation regarding matters to which the Panama Papers are relevant."

His office said it "would greatly appreciate the opportunity to speak as soon as possible" with someone from ICIJ on the matter.

The letter gave no information on the subject of the investigation, and was not a subpoena that would force the ICIJ to turn over any of the 11.5 million confidential documents leaked from Panama law firm Mossack Fonseca.

Access to the documents remains limited to a handful of international media organizations.

Reports from the ICIJ and the media groups based on the Panama Papers link a number of international leaders, including Russian President Vladimir Putin, British Prime Minister David Cameron, and leaders and top officials in Ukraine, China and Argentina, to offshore shell companies that can be used to hide wealth and launder money.

Iceland's Prime Minister Sigmundur David Gunnlaugsson was also forced to resign after the documents showed he and his wife made use of an offshore company.

Panama and Japan agreed on an early start of talks aimed at creating a bilateral framework for exchanging taxation-related information, Panamanian President Juan Carlos Varela and Japanese Prime Minister Shinzo Abe said on Wednesday.

The announcement comes after the leak of thousands of confidential documents from a Panamanian law firm earlier this month highlighted Panama's failure to cooperate in international efforts to clamp down on tax evasion by the rich and powerful. "Let me reconfirm the Panamanian government's responsible engagement with and commitment to transparency in the financial system," Varela told a joint news conference in Spanish.

"As I said in a speech made in the UN General Assembly last September, Panama intends to move toward bilateral, automatic exchange of taxation information."

Abe welcomed the agreement as a step toward improved transparency in bilateral taxation-related information, although he told Varela in their meeting that it was important to address the issue of international tax evasion in a multilateral framework.

Panama has been "wrongly" labelled a tax haven, President Juan Carlos Varela said in an interview with a Japanese news agency, defending his country amid the global "Panama Papers" scandal.

Panama is scrambling to avert redesignation as a tax haven that assists money laundering after the disclosure of the offshore dealings of many of the world's wealthy, famous and infamous.

They came to light when millions of documents covering nearly 40 years of business were leaked from the archives of the Panamanian law firm Mossack Fonseca.

The revelations have had far-reaching political consequences, already bringing down the Icelandic prime minister and Spain's industry minister, while forcing others to explain their financial dealings.

"Wrongly, we have been labelled a tax haven," Varela said in the interview in Tokyo on Tuesday with Jiji Press. "Panama is a country respectful of laws."

In the interview, Varela said that his country will establish a high-level commission, to be composed of six to eight internal and external members, within six months in order to improve transparency of its financial system.

The members are slated to include Nobel Prize-winning economist Joseph Stiglitz, Jiji reported.

Varela, who arrived in Japan on Sunday, also called for global efforts to tackle the scandal on Tuesday, insisting it is not a problem just for his country but the international financial system as a whole.

In an investment seminar in Tokyo, Varela said Panama is willing to cooperate with an Organisation for Economic Cooperation and Development initiative to share tax information, saying: "We have the door open to do it in a multilateral way."

Varela said he plans to announce a bilateral taxation scheme with Japan under OECD standards on Wednesday following his meeting with Prime Minister Shinzo Abe.

The Panamanian president's visit to Japan comes after the world's leading economies took a step last week toward denying tax evaders and money launderers the ability to hide behind anonymous shell companies.

Acting in the wake of the scandal, finance chiefs of the Group of 20 meeting in Washington on Friday supported proposals requiring authorities to share the identities of shell companies' real owners.

They also backed creating a blacklist of international tax havens that do not cooperate with information-sharing programmes.