Turkish Deputy Prime Minister Numan Kurtulmus says his country has reached the end of its "capacity to absorb" refugees but will continue to take them in.
Turkey is facing mounting pressure to open its border to the tens of thousands of Syrians, as pro-government forces push on with their offensive in the northern Aleppo province.
The United Arab Emirates has joined Saudi Arabia in saying it would consider sending ground troops to Syria to battle the Islamic State group, raising the possibility of even greater foreign involvement in the war.
Turkey says up to 35,000 Syrians have massed along the border, which remained closed for a third day on Sunday. The governor of the Turkish border province of Kilis said 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 three million refugees, including 2.5 million Syrians.
"Turkey has reached the end of its capacity to absorb (refugees)," 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."
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 on the other side of the border at 30,000.
He 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.
Opposition activists said Syrian ground troops backed by Russian airstrikes were engaged in intense fighting with insurgents around the village of Ratyan and surrounding areas north of Aleppo city.
The army has almost fully encircled Aleppo, Syria's largest city and one-time commercial centre, preparing the way for a blockade. The main supply line to the Turkish border has already been cut and many residents of the city were looking to leave, anticipating severe shortages in coming days.
The Syrian American Medical Society says there are now only four general surgeons in the city.
The European Union has urged Turkey to open its borders and EU nations have committed three billion euros ($A4.72 billion) to Turkey to help refugees, part of incentives aimed at persuading Turkey to do more to stop thousands of migrants from leaving for Greece.
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