Serbia's economic power grew rapidly in the 14th century, and Milutin's power was based on new mines, mostly in Kosovo territory. During his regin, Novo Brdo was the richest silver mine in the Balkans, while another important mines were Trepča and Janjevo. He produced imitations of Venetian coins, which contained seven-eighths of silver compared to their coins. The were banned by the Republic of Venice, but Milutin used them to wage civil war against Dragutin. Later, Novo Brdo became an internationally important silver mining site and significant strategic position, while in the 15th century, Serbia and Bosnia combined produced over 20% of European silver.