(Reuters) -Starbucks said on Thursday it would close underperforming stores in North America, including its iconic Seattle roastery, as CEO Brian Niccol presses ahead with his restructuring effort, which is expected to cost $1 billion in a bid to revive the company's flagging sales. Niccol is trying to restore the chain's "coffeehouse" feel to bring customers back to its outlets after six consecutive quarters of declining U.S. sales. Among the closed stores was Starbucks' flagship unionized location in Seattle, a large cafe with an in-house roastery, the company confirmed.