Greece said on Monday it had prevented around 600 migrants from crossing the Aegean Sea into its territorial waters from neighbouring Turkey, in the largest attempted entry this year.
A spokesman for the Greek coastguard said five sailboats and four dinghies had set off from the Turkish coast early in the morning.
"Greek patrol vessels were able to quickly locate the vessels and inform the Turkish coastguard," the spokesman told AFP.
A coastguard statement said its boats had used "visual and sound signals" to keep the potential asylum seekers out of Greek territorial waters.
All the vessels either headed back or were intercepted by the Turkish coastguard, the spokesman added.
"All the incidents occurred inside Turkish territorial waters" near the Greek islands of Chios and Samos, he said.
A migration ministry source said migration flows to the Greek islands in the first four months of 2022 were nearly 30 percent higher than in the same period last year.
Over 3,000 asylum seekers have arrived in Greece so far this year, including over 1,100 last month, according to the ministry's data.
There is also heightened migrant activity on Greece's land border with Turkey because water levels are low on the River Evros that divides the two countries, a border official said on Monday.
According to the latest official statistics from April, there are more than 2,300 asylum seekers in camps on the Greek islands of Lesbos, Samos, Chios, Kos and Leros, all near Turkey.
New camps funded by the European Union were recently completed in Samos, Leros and Kos. Others on Lesbos and Chios are to follow.
Greece regularly blames Turkey for not taking sufficient action to curb people smugglers who send out migrants in unsafe boats and dinghies from its shores, in breach of a 2016 accord with the EU.
Greece is a member of the 27-nation bloc but Turkey is not.
The Mediterranean Sea has been the centre of migrant flows from Africa and the Middle East to Europe for several years.
An acute migrant crisis peaked in 2015 as migrants, many fleeing the Syrian civil war, sought refuge in Europe.
Also on Monday, France-based non-government organisation SOS Mediterranee said its Ocean Viking ship had picked up 139 people off the coasts of Libya and Malta after sailing boats rescued two stricken vessels.
Greece's tough border controls have been dogged by accusations from rights groups that the Greek coastguard has been engaging in illegally forcing migrants to return to Turkey.
Athens has always denied that its security forces engage in illegal pushbacks.
In March, Greece's national transparency authority said a four-month investigation found no evidence of such practices.
EU border agency Frontex has also repeatedly been accused by rights groups of illegally returning migrants across EU borders.
Frontex head Fabrice Leggeri resigned last month amid an investigation by the European anti-fraud office OLAF, reportedly into alleged mismanagement.
