MOSCOW, Dec 20 (Reuters) - Alfa Group, controlled by Russian billionaire Mikhail Fridman, has boosted its stake in Russian-Dutch tyre maker Amtel-Vredestein to a blocking 26.14 percent from 17.35 percent, Alfa said on Wednesday. Alga Group said in a statement it had bought 6 million ordinary shares from the company's founder Sudhir Gupta. It gave no price for the shares.