NATO said Thursday traffickers were still trying to smuggle people across the Aegean Sea, despite an unprecedented naval mission by the alliance to help tackle Europe’s migration crisis.

NATO said Thursday traffickers were still trying to smuggle people across the Aegean Sea, despite an unprecedented naval mission by the alliance to help tackle Europe's migration crisis.

"We have seen a significant reduction in the numbers" attempting the perilous crossing from Turkey to Greece, particularly since the implementation of a deal between Ankara and the European Union, NATO chief Jens Stoltenberg said.

"But I think it is important not to end this activity too early because we still see that the smugglers are trying to get people over the Aegean," he said at a press conference during a visit to Turkey.

Former Norwegian prime minister Stoltenberg said NATO's deployment of ships and helicopters in Turkish and Greek territorial waters, launched early March, would "stay as long as needed," rather than pull out now that numbers are dropping.

He warned that leaving could "see a return of high numbers of people," adding that the alliance would "need to remain flexible because the people smugglers are shifting routes very rapidly".

According to the UNHCR, the daily average number of people arriving in Greece via the Aegean has so far been 134 in April, down from 870 in March, when the NATO civilian operation launched and the EU-Turkey deal took effect.

Under the accord, migrants who travel to the Greek islands -- a main entry point for those seeking a new life in Europe -- are being returned to Turkey in return for billions in EU aid.

The EU also promised to resettle one Syrian refugee for every Syrian taken back by Turkey, to grant visa-free travel to Turks within the border-free Schengen Zone and to reassess Turkey's stalled EU membership bid.

Stoltenberg said the NATO mission was creating "an additional platform for cooperation between Turkey and Greece and between Turkey and the EU".