The United States and the United Kingdom are reviewing more than $20 billion in cryptocurrency transactions conducted through a Russian virtual exchange, according to people familiar with the matter. It is part of a joint effort to crack down on support for Putin's efforts to evade sanctions during his military campaign in Ukraine. The transfer under review was made through Moscow-based cryptocurrency exchange Garantex, using the U.S. dollar-pegged cryptocurrency Tether. They said anonymously that the transfers were made after Garantex was sanctioned by the United States and the United Kingdom for allegedly facilitating financial crimes and illegal transactions.