Big oil companies and Wall Street banks have stopped trading crude with Libya in response to sanctions against the country, threatening a near-shutdown of exports from the North African country and driving oil prices even higher.

Morgan Stanley, which buys Libyan oil for its clients, has stopped buying because of sanctions announced last month, according to a person familiar with the matter. ConocoPhillips Co. said it isn't exporting any of the 46,000 barrels a day of oil it normally produces in Libya. Exxon Mobil Corp. also...