Greece has threatened not to sign off on the final conclusions of an EU summit on migration and Britain's membership, holding out for assurances EU states will not shut borders over the migrant crisis, government officials said on Friday. "We can't agree unless there is a clear statement in the conclusions that there won't be any unilateral (migrant) action until the next summit. Although not related to the issue of Britain's renegotiation of its terms with the EU - a move ahead of a referendum on whether Britain stays in - the Greek move could complicate an already delicate situation.