At least 50 people have died in fresh fighting between government troops and Tamil Tiger rebels in northern Sri Lanka, the island's defence ministry said Monday.  The fighting, which raged on Sunday, raises the number of rebels killed by security forces since the start of the year to 3,774, while 286 soldiers have died in combat during the same period, according to ministry figures.  Meanwhile, Sri Lanka's capital was on high alert, with 1,500 extra policemen in Colombo during the weeklong festivities, police officials said.