By David Lawder, Asif Shahzad and Ariba ShahidWASHINGTON/ISLAMABAD, Aug 24 (Reuters) - The United States was set to warn countries to cut ties with Iran or risk their businesses being cut off from the dollar-based financial system when it unveils the "greatest financial offensive ever" on Monday, a source told Reuters.

Iran's rial slumps to record low ahead of sanctions announcement; Tehran charges measures prove US failed in war, says it issued warnings against cooperating with pressure campaign

By Yasmine Ghania and Kanishka SinghCAIRO/WASHINGTON, Aug 24 (Reuters) - The U.S. threatened Iran with what it called "the greatest financial offensive ever marshalled" as it…