More bloodshed in Iraq: American-led international troops in Iraq were locked today in the fiercest fighting since the fall of Saddam Hussein a year ago, waging a two-front war against Sunni Muslim insurgents west of Baghdad and a ferocious and fast-spreading Shiite uprising in the south and center of the country. American marines fired rockets at a wall surrounding a mosque in Falluja, west of Baghdad, killing more than two dozen people, news services reported, quoting witnesses. The American military would not confirm the accounts, which are certain to aggravate the violent opposition movement. The extent of American casualties in fighting that stretched from the far south to Kirkuk in the north remained unclear, though military spokesmen confirmed today that 12 American marines died on Tuesday in a seven-hour assault by gunmen on an American base in the Sunni-dominated city of Ramadi, 70 miles west of the capital.