WASHINGTON, Feb 9 (Reuters) - The United States on Thursday imposed sanctions on companies it accused of playing a critical role in the production, sale and shipment of Iranian petrochemicals and petroleum to buyers in Asia, as Washington increases pressure on Tehran. Treasury Department in a statement said it imposed sanctions on six Iran-based petrochemical manufacturers or their subsidiaries and three firms in Malaysia and Singapore over the production, sale and shipment of hundreds of millions of dollars worth of Iranian petrochemicals and petroleum. See here for a complete list of exchanges and delays