Iran is vowing to block a key passage for the world’s oil supplies if the U.S. proceeds with new economic sanctions. On Tuesday, a top Iranian official said Iran would close off the Strait of Hormuz if the United States fully implements a measure that would sanction Iran’s oil exports, a major source of Iranian government revenue. About one-fifth of the world’s oil is said to pass through the Strait. The sanctions were passed as part of a congressional measure targeting Iran for its alleged nuclear activities and now await President Obama’s signature.