The number of refugees arriving in the EU from Turkey has not dropped sufficiently since Brussels agreed to provide Ankara with €3bn in aid in return for a crackdown, the EU's top official responsible for tackling the migration crisis has said.
Frans Timmermans, the European Commission's first vice-president, said on Thursday that he would fly to Turkey this weekend to discuss the issue with Turkish authorities, adding that Ankara had to improve its efforts in order to live up to the terms of November's joint action plan.
"We are all committed as part of the joint action plan to bring the figures substantially down. It's quite clear that over the last couple of weeks the figures have remained relatively high," Mr Timmermans said at a news conference in Amsterdam with Dutch and EU leaders. "So there's still a lot of work to do there."
His remarks come amid mounting concern in Berlin that its Turkey-focused strategy aimed at halting the influx, which brought more than 1m refugees to Germany last year, is not bearing fruit fast enough.
The strategy has come under renewed scrutiny following police reports from the German city of Cologne that men of "north African and Arabic" appearance assaulted dozens of women during the recent New Year's festivities.
Anti-immigrant groups have seized on the incident, arguing that it showed the failure of German chancellor Angela Merkel's refugee policy.
According to UN figures, the number of refugees arriving in Greece frequently exceeded 3,000 a day during the past two weeks. Although that is below the numbers of early December, it is still enough to overwhelm Greek and EU authorities. Many leaders also believe the dip is due to bad weather rather than Turkish efforts.
"Our impression is that the drop is predominantly linked to the weather, namely a stormy sea in the Mediterranean," Thomas de Maizière, German interior minister, said this week.
Mr Timmermans, who was in Amsterdam to kick off the Netherlands' spell at the helm of the EU's six-month rotating presidency, said Brussels would need to work with Ankara at "improving the effectiveness of their operations".
"We are a long way from being satisfied and we will continue our efforts to make sure we deliver the results we agreed with Turkey," he added.
But Ankara has long made clear the migrant flow to Europe might continue.
Ahmet Davutoglu, Turkey's prime minister, said when the deal was signed that he could offer "no guarantees", saying it would depend on events in Syria. Turkish officials say the funding promised by the EU was to sustain migrants within Turkey and is not contingent on stemming numbers.
Gerald Knaus, head of the European Stability Initiative, a think-tank, said it had been unrealistic to imagine the deal would bring down refugee numbers. "It was obvious to everybody three months ago that it wouldn't work. Both sides are to blame for that, because they didn't negotiate seriously," he said.
The only solution, he added, would be if Turkey committed itself to taking back refugees who had left the country for Greece and whose asylum applications had been rejected.
Additional reporting by Alex Barker in Brussels and Guy Chazan in Berlin.