LONDON — The leak of millions of records on offshore accounts claimed its first high-profile political casualty Tuesday as Iceland’s prime minister stepped aside amid outrage over revelations he had used such a shell company to shelter large sums while Iceland’s economy was in crisis. Icelandic leader Sigmundur David Gunnlaugsson is the first major figure brought down by the publication of the names of rich and powerful people linked to the leaks, dubbed the Panama Papers. China and Russia, meanwhile, took the opposite approach, suppressing the news and rejecting any allegations of impropriety by government officials named in the leak of more than 11 million financial documents from a Panamanian law firm. Officials in Ukraine, Argentina and other countries are also facing questions about possibly dubious offshore tax-avoidance schemes. The reports are from a global group of news organizations working with the Washington-based International Consortium of Investigative Journalists.