The White House is considering a ban on all U.S. trade with Iran to punish efforts to build a nuclear arsenal there. ButTIME Diplomatic correspondent J.F.O. McAllistersays the Clinton Administration previously has been opposed to an all-out embargo because the business likely would be diverted to firms in other countries that compete with American companies. McAllister expects President Clinton to opt for narrower restrictions. Clinton is expected to move swiftly on various options, including proposals to ban sales of computers and equipment with military applications to Iran, or toban importing oilfrom Iran for sales abroad -- a $4 billion market. Purchasing oil from Iran for sale on the U.S. market already is prohibited.