The US seizure of a Russian-flagged oil tanker in the North Atlantic dramatically raises the stakes in Washington's naval blockade of Venezuela, blowing apart an apparent Kremlin strategy to reflag its shadow fleet ships to offer them protection and stoking wider tensions with Moscow. Some 21 flagless vessels have adopted Russia's flag since the United States seized a shadow fleet tanker called the Skipper in the Caribbean on December 10, according to data from Windward, a maritime intelligence company. That was followed by US President Donald…