NATO said Thursday it had no plans to wind up a mission to stop migrant traffickers in the Aegean Sea and warned smugglers were changing routes very quickly.

Alliance chief Jens Stoltenberg said traffickers were still trying to smuggle people from Turkey to Greece, despite the apparent success of an unprecedented naval mission by NATO 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, he admitted.

"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.

He said NATO's deployment of ships and helicopters in Turkish and Greek territorial waters, launched in 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".

- 'Platform for cooperation' -

According to the UNHCR, the daily average number of people arriving in Greece via the Aegean has reached 134 so far 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".

Information gathered from the operation was passed on to Turkey, Greece and the European border agency Frontex and was helping Ankara "take action to help break the business model" used by smugglers, he said.

But critics of the EU-Turkey deal have warned that it is likely to merely see traffickers attempt new routes or return to the tried and trusted but very dangerous crossing from the coast of Africa to Italy.

The dangers faced by migrants on that route was underscored on Wednesday after reports emerged of a shipwreck in the Mediterranean in which some 500 people were feared to have drowned.

More than one million people crossed clandestinely to Europe in 2015 while some 179,000 have made the trip since the start of this year, according to UNHCR figures.

More than 3,700 people died in 2015 trying the make the perilous crossing over the Mediterranean, with 761 recorded as dead or missing this year.