WASHINGTON -- Russian prosecutors say they plan to ask U.S. authorities for assistance in their investigation of British-American financier William Browder, accusing him and New York investment fund Ziff Brothers of defrauding the Russian treasury of up to 1 billion rubles in taxes.



The planned appeal, announced May 19 by the Prosecutor-General's Office, opens a new front in Russia's mounting international campaign against Browder as Moscow seeks to undermine support for a U.S. law he lobbied vigorously for that punishes alleged Russian rights abusers.



Prosecutor-General's Office spokesman Aleksandr Kurrenoi said in a statement that Browder's investment vehicle, Hermitage Capital, and Ziff Brothers may have violated U.S. laws in what Russian authorities call the illegal acquisition of 200 million shares of state-owned energy giant Gazprom between 1997 and 2005.



Browder has repeatedly dismissed the allegations as baseless, and Interpol has previously rejected Russian requests for his arrest in connection with the accusations, calling Russia's aims in the matter "predominantly political in nature."



Kurrenoi alleged that the Gazprom shares were later transferred to a Cypriot firm, with the end result that 1 billion rubles was underpaid to tax authorities. That was equivalent to around $37 million at 2006 exchange rates.



He alleged that the scheme was orchestrated by Browder and his associate, U.S. lawyer Jamison Firestone, as well as Ziff Brothers, a New York-based firm that was one of the largest hedge funds of its kind before it closed down two years ago.

"At this moment, the Prosecutor-General's Office is drafting a query for international legal assistance to our counterparts in the U.S.," Kurrenoi said in a statement.

Browder told RFE/RL that the allegations have an "obvious political direction" and "lack of merit," noting the Interpol rejection of Russia's request for an arrest warrant in the matter.

A representative for Ziff Brothers did not respond to a request for comment, and Firestone could not be immediately reached.



The Russian government and its allies in recent months have stepped up their pressure on Browder in an international campaign challenging the U.S. Magnitsky Act, a 2012 law that introduces visa bans and asset freezes for Russians alleged to be involved in the death of whistle-blowing accountant Sergei Magnitsky.



Magnitsky, who worked for Browder, accused Russian law enforcement and tax officials of a massive tax fraud scheme prior to his November 2009 death in a notorious Moscow jail. His family and friends say he was tortured, beaten, and denied critical medical treatment shortly before he died.



Browder subsequently helped lobby for the Magnitsky Act in the U.S. Congress. After it was enacted in 2012, the law sanctioned 18 people linked to Magnitsky’s death, the original fraud, and other unrelated cases. That number has since grown to 39.



The law infuriated Moscow, and Russian officials have repeatedly attacked Browder directly and indirectly. A television documentary shown last year on the Russian television channel NTV, which is owned by a Gazprom subsidiary, repeated earlier charges that the tax fraud uncovered by Magnitsky was perpetrated by Browder.



U.S. prosecutors have also moved to seize real estate in Manhattan owned by a company called Prevezon that allegedly received some of the laundered proceeds of the original Russian tax fraud.



That case, which was scheduled to go to trial in January, has been put on hold while a U.S. appeals court hears arguments about whether Prevezon’s lawyers should be removed from the case because they earlier did work for Browder and Hermitage. A hearing is scheduled for June 9.



Supporters of Browder in the U.S. Congress, meanwhile, have sought to broaden the Magnitsky Act to give the U.S. president the authority to sanction any person who commits abuses against human rights activists or legal whistle-blowers anywhere in the world.



A U.S. congressional committee this week voted to back the measure, but only after one lawmaker tried -- unsuccessfully -- to strike Magnitsky's name from the bill, saying it would needlessly antagonize the Kremlin.