BRUSSELS - European Council President Donald Tusk warned Turkey on Tuesday that it will only win concessions from the EU, such as easier travel visas, if it successfully reduces the flow of refugees reaching Europe.In a letter to EU leaders setting the agenda for a summit he will chair in Brussels on Thursday, Tusk noted the start this month of negotiations on the migrant crisis with Turkey, which EU officials say wants more visa waivers, more EU funding and progress on its longstanding application to join the bloc."An agreement with Turkey makes sense if it effectively reduces the inflow of refugees," Tusk wrote on the eve of a visit by senior European Commission officials to Ankara. "Concessions will only be justified when this goal is achieved."Tusk, a conservative former prime minister of Poland, is trying to steer the divided 28-nation Council toward consensus on proposals put forward by the executive Commission. He visited Turkey last month and hosted Turkish President Tayyip Erdogan on a visit to Brussels last week during which the Commission set out various ways in which Ankara could help control migration.Germany, France and others are reluctant to accept populous, Muslim Turkey into the EU and raise human rights concerns over Erdogan's policies toward, among others, Kurds and the media.Tusk warned of a "long march" toward greater cooperation with countries in the Middle East that are hosting millions of Syrian refugees and highlighted how Russia's new military action in Syria, at odds with Turkish calls for EU support to establish "safe zones" in the north, added to complexities in the region."We must be ready for spring and the threat of bigger waves flowing to Europe," he said, noting warnings from regional leaders that "millions" more could start heading for Europe."As exaggerated as this opinion may sound, it is our obligation to be prepared for all scenarios," Tusk said. He said leaders should consider changes to the Dublin system, which makes frontier states responsible for asylum, and strengthen EU borders, possibly by creating a Union border guard force.Tusk, despite the crisis aims to end the summit, which begins at 4 p.m. (1400 GMT), without resuming for a second session on Friday, officials said.The meeting will also "take stock of the next steps" in negotiations with Britain on changes it wants before holding a referendum on membership by the end of 2017.