NAIROBI (Reuters) - The average price of top Kenyan tea weakened for a fifth straight week to $3.63 per kilogramme from $3.74 at last week's sale, traders said on Tuesday. The east African nation is the world's largest exporter of black tea, an important source of its foreign exchange. Africa Tea Brokers (ATB) said in a market report the Best Broke Pekoe Ones (BP1s) fetched $2.67-$4.58 per kg, from $2.65-$4.82 per kg attained at last week's sale. Brighter Pekoe Fanning Ones (PF1s) sold at $1.96-$2.54 per kg, compared with $2.00-$2.68 per kg at the last auction. ATB said 21.76 percent of the 126,452 packages or 8.2 million kgs were unsold, compared with 29.4 percent that went unsold from the 118,979 packages or 6.97 million kgs offered for sale last week.