The EU border agency Frontex said on Tuesday it had sent 293 guards and 15 boats to the Greek islands to help Athens deal with migrant pressure.
The deployment, which Frontex said began on Monday, comes at the request of Greece but it is smaller than the government had hoped.
The number of guards will be increased to over 400 over time, Frontex said in a statement, and there will be more boats and technical equipment as part of a fast-intervention mechanism called Poseidon Rapid.
Frontex had asked Greece's EU partners to provide 600 border experts after a surge of migrants in August, especially from Syria, but they have been slow to respond.
Poseidon Rapid allows Frontex to circumvent the need for national decision-making, forcing members to contribute immediately.
"This is what Frontex can provide," agency spokeswoman Ewa Montcure told AFP, adding that migrant flows had been slowing down recently.
But estimates say every day thousands of migrants and refugees are still landing on Greek islands from the nearby Turkish coast. Frontex said it believes the total for December will be 80,000.
The number of guards being deployed "is still below what we expected", a source in Greece's migration ministry said.
The migration minister, Iannis Mouzalas, had asked for 1,600 guards to be sent to Greece, which has been the entry point into the EU for over 80 percent of this year's one million arrivals.