Sandstorms and high winds slowed the advance of a major U.S. and British force toward Baghdad Tuesday. Heavy fighting continued in pockets farther south as the war in Iraq moved through its sixth day. Marines engaged in a firefight with regular Iraqi forces and irregular militia units around the town of Nasiriyah. The U.S. Central Command in Doha, Qatar said the Marines had sustained some casualties, but did not elaborate. Meanwhile, British officials said there were