By Gabriela Baczynska and Robert-Jan Bartunek BRUSSELS (Reuters) - European Union ministers on Thursday raised the prospect of further tightening unilateral border controls unless there was a sharp drop in the number of migrants arriving from Turkey by the time of an EU-Turkey summit on March 7. Seven European states have already reinstated border controls within the cherished but creaking Schengen free-travel zone, putting huge strain on Greece, which can no longer wave the tide of arrivals from Turkey onward through the Balkans. More states said they would follow suit unless a deal promising Turkey 3 billion euros ($3.3 billion) in help to house refugees from the Syrian war in return for preventing them travelling on to Europe yielded fruit before the summit.