Rafah, May 19 - Under heavy cover fire from helicopters, Israeli troops combed this refugee camp for weapons and gunmen Tuesday in the biggest Gaza offensive in years. Twenty Palestinians were killed, including two teenagers shot as they gathered laundry. The death toll was the highest one-day total since 35 Palestinians were killed in the West Bank city in Ramallah on April 5, 2002. International condemnation mounted against the operation, and the United States said it was asking Israel for "clarification." The United Nations and European Union demanded an end to the incursion, which Israeli security officials said would last at least a week. In Rafah, a crowded camp of 90,000 people near the Egyptian border, Palestinian families sought refuge from the rocket and machine-gun fire in the innermost rooms of their homes. Last week, Israel destroyed about 100 houses, making more than 1,000 Palestinians homeless. Bureau Report