Turkey has reached the end of its "capacity to absorb" refugees but will continue to take them in, Turkey's deputy premier said.
His country is facing mounting pressure to open its border, where tens of thousands of Syrians fleeing a government onslaught have arrived.
Turkish authorities say up to 35,000 Syrians have massed along the border, which remained closed for a third day on Sunday.
The governor for the Turkish border province of Kilis said on Saturday that Turkey would provide aid to the displaced within Syria, but would only open the gates in the event of an "extraordinary crisis".
Deputy Prime Minister Numan Kurtulmus told CNN-Turk television that Turkey is now hosting a total of 3 million refugees, including 2.5 million Syrians.
"Yes, Turkey has reached the end of its capacity to absorb (refugees)," Mr Kurtulmus said. "But in the end, these people have nowhere else to go. Either they will die beneath the bombings and Turkey will ... watch the massacre like the rest of the world, or we will open our borders."
Mr Kurtulmus said some 15,000 refugees from Syria were admitted in the past few days, without elaborating. He put the number of refugees being cared for at the other side of the border at 30,000.
"At the moment, we are admitting some, and are trying to keep others there (in Syria) by providing them with every kind of humanitarian support," Mr Kurtulmus added. "We are not in a position to tell them not to come. If we do, we would be abandoning them to their deaths."
The deputy premier did not explain why the Turkish border gate at Oncupinar, opposite the Bab al-Salameh crossing in Syria, was being kept closed or why tens of thousands of refugees were not immediately being let in.
On Saturday, the European Union urged Turkey to open its borders at a meeting between EU and Turkish officials in Amsterdam, saying it was providing aid to Ankara for that purpose. EU nations have committed 3 billion euros to Turkey to help refugees, part of incentives aimed at persuading Turkey to do more to stop thousands of migrants from leaving for Greece.
Mr Kurtulmus estimated that - "in the worst case scenario" - as many as one million more refugees could flee the Syrian city of Aleppo and its regions.
The war between Syrian president Bashar Assad's government and Syrian rebels began in 2011. It has killed over 250,000 people and forced millions to flee the country.