WASHINGTON -- American lawmakers are demanding answers from their national government after a foe of Vladimir Putin, born in the United States, was allegedly denied entry into his native United States, in a case with cross-border repercussions.

Bill Browder, born in the U.S. but now a British citizen, says he has been placed on an international police watch-list and blocked from a flight to the U.S., and he accuses the Russian government of being behind it.

He had just successfully lobbied Canada to become the latest country to adopt a so-called Magnitsky law that targets corrupt officials of the Putin regime, and he is accusing the Russian government of retaliating by placing his name on an Interpol watch list.

"United (Airlines) wouldn't let me board a flight," Browder tweeted Monday. "Unless Interpol lifts this notice, I will be arrested at any international border I cross on Putin's orders."

The developments are the latest twist in a dark saga involving him and the Russian leader.

Browder was a major hedge-fund manager in Russia who complained about oligarchs stealing from companies he'd invested in. Initially, in the early 2000s, Putin was his ally -- Browder's complaints about corruption would be followed by arrests.

The arrests stopped. Browder accuses Putin of becoming personally involved in corruption schemes, building a massive multibillion-dollar fortune, and turning on him. A crusading young lawyer working to investigate corruption for him, Sergei Magnitsky, was suddenly arrested by some of the very people he'd investigated. He was beaten repeatedly in prison and eventually died.

Browder has since made it his life's mission to avenge his friend's death. He has succeeded in getting several countries to pass laws in Magnitsky's honour. Canada adopted its law a few days ago.

The Canadian law allows authorities to block financial services for foreigners complicit in extrajudicial killings, torture, or other human-rights violations in efforts to protect a government that abuses basic rights.

Putin angrily responded to a similar law in the U.S. by stopping adoptions of Russian orphans. The issue has been brought up at the highest levels. The White House says President Donald Trump's son, son-in-law, and campaign manager discussed it with a Russian lawyer last year at Trump Tower.

Browder says he's often been the victim of Russian attempts to get him placed on Interpol wanted lists and says that this time he's been accused of murder -- of killing his friend Magnitsky.

American lawmakers from both parties are demanding answers. Sen. John McCain called Browder a champion of anti-corruption fighters in Russia and called for an immediate U.S. review of his status.

"We relied on his expertise and support as we led the effort to pass the Magnitsky Act," McCain said in a statement.

"Mr. Browder's work has helped to remove corrupt actors from our financial system and enhance accountability measures with respect to the U.S. relationship with the Russian Federation -- it would be unfortunate if the U.S. decided to bar him based on a decision by those same Russian officials who have been targeted by this important legislation."

It's unclear how his travel was denied.

Browder said he had an electronic document allowing travel from visa-free countries, via the Electronic System for Travel Authorization. In a statement, the U.S. State Department said, "We have no record of this individual ever applying for a U.S. visa."

The statement noted that many United Kingdom citizens are eligible to travel via the visa waiver, but if they lose eligibility to use the visa-waiver program they must apply for a visa at a U.S. consulate or embassy.