The European Union on Sunday agreed with Turkey's government for Ankara to take steps to cut the flow of migrants into Europe in exchange for EU cash and help with its bid to join the 28-nation bloc.

EU leaders hailed the agreement as a key step toward substantially reducing the number of asylum seekers entering the bloc, while Turkey's Prime Minister Ahmet Davutoglu said Sunday's summit marked a historic new beginning in the often fraught relations between Brussels and Ankara.

Yet the continued lack of trust on both sides remained evident, as EU leaders made it clear there would be no shortcut in Turkey's long-stalled bid to join the bloc. "The issue hasn't changed," French President Franç ois Hollande said after leaving the summit to return to Paris for global climate talks. "There is no reason either to accelerate or to slow it down."

And the Turks couldn't say how effective the agreement would be in reducing the number of the migrants and refugees entering the EU via Turkey.

EU officials have said cooperation with Turkey is the best way to reduce migrant flows, arguing that Ankara was very effective in previous years in preventing the outflow of refugees from the country.

Alongside fresh efforts to tighten their external borders, EU officials hope the Turkey agreement can help turn the tide in the bloc's migration crisis, the biggest since the aftermath of World War II.

Under Sunday's agreement, Turkey would increase patrols in the Aegean Sea and on the land borders with Greece and Bulgaria, as well as crack down on human-trafficking gangs. Turkey also agreed—starting next year—to implement an agreement to take back migrants whose asylum claims are denied by EU countries.

In exchange, EU leaders pledged to provide an "initial" €3 billion ($3.19 billion) to Turkey to help it handle the more than two million refugees in the country. Much of that money would go directly to groups helping refugees in Turkey or to programs set up by the Turkish government to house, train and integrate migrants.

EU leaders also promised to open negotiations on a new chapter in Turkey's bid to join the bloc—a move likely to be approved on Dec. 14—and to hold twice-yearly summits with Turkey's leaders. They pledged to speed work on Ankara's bid to win visa-free access to the EU for its citizens.

German Chancellor Angela Merkel had organized pre-summit talks on Sunday on a proposal Berlin hopes could see tens of thousands of Syrian refugees in Turkey come legally to the bloc. Ms. Merkel said after the summit that discussions were still in their early stages, but the idea was for a voluntary program in which EU countries willing to accept migrants can receive and settle an agreed number of Syrian refugees. "We didn't name a particular figure," Ms. Merkel said. "I don't think everyone would participate."

Still, it appeared that substantial efforts would be required to turn Sunday's agreement into reality. European Council President Donald Tusk said the EU will closely watch Turkey's implementation of the deal and will review Ankara's actions on a monthly basis. EU governments are also still at loggerheads over who would pay the €3 billion Turkey is to receive for its cooperation.

Moreover, Turkey must complete dozens of EU requirements to win a recommendation for visa-free access to the bloc by autumn of 2016. Even then, a final decision will need backing of all 28 member states.

Meanwhile, Mr. Davutoglu acknowledged he couldn't promise the number of migrants heading into Europe via Turkey would fall. "Nobody can guarantee a drop," he said of the refugees heading west from war-torn Syria. "I am sure if we implement our joint action plan…there will be less pressure on Turkey and the EU."

In recent months, Turkey has become the main crossing point for refugees from Syria and elsewhere to enter the EU, given the short sea journey from Turkey to Greece. This year, more than 700,000 migrants have arrived in the EU via Turkey, according to the International Organization of Migration.

The EU has struggled to cope with the influx of migrants. The recent terror attacks in Paris and the lockdown in Brussels have heightened concern about who is crossing Greece's porous border with Turkey.

Mr. Davutoglu said after the summit that 2016 could be a key year in Ankara's bid to join the bloc. "Membership is a strategic objective for us. That's going to speed up again in 2016," he said. Turkish membership could "be a reality in the coming years."

However, the past decade of negotiations over membership hasn't been encouraging. In that period, the EU and Turkey have opened talks on 14 of 35 chapters that must be negotiated for the country to enter the group. Few EU officials believe the myriad technical and political hurdles slowing progress in the past are about to disappear.

Part of Sunday's agreement would require the EU to carry out preparatory work on five currently blocked chapters during the first three months of 2016. That would mean talks could theoretically start on key issues later next year, including justice, energy, defense and foreign policy.

But in a reminder of the obstacles accession talks will continue to face, Cyprus, whose government isn't recognized by Ankara, rejected the mention of those five specific chapters in Sunday's joint EU-Turkish declaration by the leaders.

Instead, European Commission President Jean-Claude Juncker has written a side-letter to the Turkish prime minister outlining the planned work. Ultimately, any of the EU's 28 member states can veto the opening of a new accession chapter.

While the refugee crisis has strengthened Ankara's hand in negotiations with Europe, domestic developments in Turkey continue to cause tensions.

Two prominent Turkish journalists were jailed Thursday, pending trial, in connection with a report alleging Turkey shipped weapons to Syrian rebels, an accusation that the government had repeatedly denied. The arrests of the journalists follow the government's seizure of opposition media outlets before elections in November.

Mr. Juncker said after the summit that the EU won't shelve its criticism of Turkey's human rights record even as Ankara and Brussels seek to strengthen ties.

Tom Fairless contributed to this article.

Write to Laurence Norman at laurence.norman@wsj.com and Emre Peker at emre.peker@wsj.com

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(END) Dow Jones Newswires

November 29, 2015 20:25 ET (01:25 GMT)