Dozens of refugees and migrants have reached the Turkish borders shared with the EU after the former said it would no longer prevent migration to the bloc.
In Evros, northeastern Greece, a police official said refugees and migrants were heard shouting "open the borders" as they reached the area on Friday.
Video journalist Savvas Karmaniolas on Lesbos also recorded two dinghies of refugees reaching the island.
Both Greece and Bulgaria say they have stepped up their patrols along the Turkish land borders.
Bulgarian Prime Minister Boyko Borissov said he had "urgently deployed" the army, national guard and border police to respond to the situation and to "beat off a possible migrant influx".
It comes after Turkey's comments prompted hundreds of refugees and migrants to head toward EU borders overnight on Thursday.
Meanwhile, the EU has called on Turkey to "honour its commitments" to its 2016 pact on curbing migration to the bloc.
Turkey's action came as it triggered an emergency NATO meeting following a deadly airstrike in Idlib, Syria, that killed at least 33 Turkish soldiers.
It was the largest death toll for Turkey in a single day since it first intervened in Syria in 2016 against Russia-backed Syrian forces.
Turkey hosts some 3.6 million Syrians and made a deal with the EU to step up efforts to halt the flow of refugees — but President Recep Tayyip Erdogan has repeatedly threatened to “open the gates."
Emergency NATO meeting over Idlib airstrike
NATO Secretary-General Jens Stoltenberg called for an immediate return to a ceasefire in Syria and reaffirmed support for Turkey following emergency talks at the latter's request.
He said allies were continuing to search for ways to support Turkey following the strike and promised increased air defences against missile strikes from Syria.
The emergency meeting, held on Friday morning in Brussels, was held under Article 4 of NATO’s founding treaty, which allows any ally to request consultations if it feels its territorial integrity, political independence or security is threatened.
Apart from providing some aerial surveillance over Syria, NATO plays no direct role in the conflict-torn country, but its members are deeply divided over Turkey’s actions there, and European allies are worried about any new wave of refugees arriving.
Vladimir Putin spoke on the phone to Erdogan on Friday morning and pledged to avoid "such tragedies" in future and said Moscow would do "everything to provide security for Turkish soldiers" deployed to Syria, according to foreign minister Sergei Lavrov.
Erdogan testing NATO patience
Turkey’s invasion of the north of the conflict-torn country — along with the criticism and threats of sanctions brandished by fellow allies at Ankara over the offensive — has come close to sparking a crisis at the military alliance.
France, in particular, has tried to launch a debate on what Turkey’s allies should do if Ankara requests their assistance under Article 5 of the Washington Treaty — which requires all allies to come to the defence of another member under attack — but that discussion has not happened.
The allies are extremely reluctant to be drawn into a conflict of Turkey’s making, and particularly because Erdogan has used up a lot of goodwill by testing his fellow NATO members' patience for quite a while.