U.S. President Donald Trump pledged on Monday that firms doing business with Tehran would be barred from the United States as new U.S. sanctions against Iran took effect in spite of pleas from Washington's allies. FILE PHOTO: U.S. President Donald Trump speaks to reporters after signing a proclamation declaring his intention to withdraw from the JCPOA Iran nuclear agreement in the Diplomatic Room at the White House in Washington, U.S. May 8, 2018.