A U.S. law on conflict minerals is curbing African warlords' presence around mines in Congo, campaigners say, but its full impact remains unclear, with most firms failing to pinpoint the origin of their metals by a June deadline...Under the Dodd-Frank financial reform law, U.S. companies must try to establish the origin of four metals often used by rebel groups in the area to finance their activities...Only five percent of firms making filings by a June 2 deadline traced the conflict status of the minerals used in their products...A new certification scheme organised by the tin industry body ITRI is being rolled out in North Kivu after earlier projects in Congo's copper-rich southeastern provinces of Katanga and South Kivu...Some analysts say the law's impact has been overstated...Campaigners are also urging the European Union to strengthen a conflict minerals proposal released in March, making it mandatory instead of voluntary...'When the conflict minerals law was first passed, there were fears that it would lead to companies boycotting the region's minerals, and some firms initially moved in that direction, but that is less of a problem now'