Dec 28 (Reuters) - The U.S. State Department has approved the possible sale of Volcano anti-tank mine-laying systems to Taiwan for an estimated $180 million, the Pentagon said on Wednesday.U.S. law requires the executive branch to notify Congress of potential arms sales that are over a certain amount.But these notifications are usually not made unless lawmakers have given the State Department and Pentagon informal approval to move ahead.Taiwan's defence ministry said in a statement that the sale would take effect in about a month and that the system would help boost the island's "asymmetric warfare" capacity to make its forces more agile."The Chinese Communist Party's frequent military activities near Taiwan have posted severe military threats to us," the ministry said, adding that continuous U.S. military sales are the "cornerstone of maintaining regional stability and peace.""