The U.S. Navy has boarded ``a handful'' of ships in its week-old effort to halt Iraqi shipping, the Pentagon said Thursday. Spokesman Pete Williams, asked to be more specific, said ``a handful would have to be less than 10.'' He said he could not give a definitive number nor say when or where the boardings occurred. In recent days, the United States has been urging swift approval of a U.N. resolution allowing military force to support the international body's sanctions against Iraq over its invasion of Kuwait, but the Soviet Union has been counseling that caution be used before military force is applied. At a Pentagon briefing, Williams said he was not aware of any more shots having been fired, but said ``dozens'' of ships were involved in the interdiction operations. ``We continue to monitor ships,'' Williams said. Ships belonging to the two aircraft carrier battle groups in the Red Sea, one carrier battle group in the Gulf of Oman and the ships of the Middle East Task Force in the Persian Gulf are involved in the operation. Boarding parties have been issued shotguns, pistols and specialized instructions on how to board and search vessels suspected of carrying cargo bound to or from Iraq. The sailors carry plastic cards in French, Arabic and English telling the ships' crews to remain calm and that no harm will come to them if they cooperate with the U.S. boarding parties. Williams said there was a ``high degree of voluntary compliance'' with the effort, and that ``many ships have turned back.'' Again, he said he had no specific number. Williams said the two Iraqi tankers involved in Saturday's shooting incident _ the first of the operation _ were ``still being shadowed'' by U.S. Navy ships. The fast frigate USS Reid fired six warning shots across the bow of the Iraqi-flagged vessel Khanaqin in the Gulf of Oman after it refused repeated requests to halt. In a separate incident, the USS Bradley fired three warning shots across the bow of another Iraqi tanker. The Khanaqin unloaded ``a portion of its cargo'' in Aden, Yemen, on Tuesday and has left port, Williams said. ``The Yemenis have assured the United Sates that the off-loading stopped before it was completed,'' the spokesman said. Williams referred reporters to the State Department when they asked whether it had been determined that the cargo involved items that would break the U.N.-sanctioned embargo against Iraq. Asked whether any other ships had been challenged and had sailed on, Williams replied: ``Not that I know of.''