Turkey is under pressure to admit 40,000 Syrians who have been camped on the border between the two countries after fleeing the biggest Syrian government advance in months.
With help from Russian air strikes, forces loyal to Syrian president Bashar al-Assad have stepped up their assault on Aleppo, Syria's biggest pre-war city, where opposition-held areas are close to being encircled.
Air strikes have also targeted towns and villages to the north of the city, apparently in an attempt to cut Aleppo off from Turkish supply lines.
The capture of eastern Aleppo city, where 350,000 people live, would be a major strategic prize for Mr Assad and underline how Russian intervention has tipped the balance in favour of his forces while reversing gains made by the rebels last year.
"In some parts of Aleppo, the Assad regime has cut the north-south corridor . . . Turkey is under threat," Turkish president Tayyip Erdogan was quoted by the Hurriyet newspaper.
Exodus of 40,000
The fighting of the past week has resulted in an exodus of 40,000 towards the Turkish border, but since Turkey closed the only official crossing in the region last Thursday the refugees have been forced to camp on the Syrian side.
Aid lorries and ambulances entered Syria on Sunday, while aid officials said Turkish aid agencies had set up shelters on the Syrian side. "If needed, we will let those brothers in," Mr Erdogan was quoted as saying.
Turkey has given refuge to civilians fleeing Syria throughout the conflict, but is coming under growing pressure from the United States to secure the border more tightly, with further pressure from Europe to stem the onward flow of migrants. It is already sheltering more than 2.5 million Syrians, the world's largest refugee population.
While areas to the northwest of Aleppo are held by Syrian opposition forces and Kurdish groups, the territory to the northeast is held by Islamic State.
The Syrian Observatory for Human Rights, which monitors the war, said there had been fierce clashes in that area, and state media said government forces had wrested a strategic hill in the eastern Aleppo countryside from Islamic State.
Mr Erdogan also criticised the US for backing Syrian Kurdish fighters that Turkey classifies as terrorists.
Last week, Brett McGurk, President Barack Obama’s envoy for the international coalition against Islamic State, visited the Syrian town of Kobane, where Kurds fought back a siege by Islamic State near Turkey’s border last year. “How can we trust you?” Mr Erdogan said of Mr McGurk’s visit. “Is your partner me, or is it those terrorists in Kobane?”
Ground troops
Meanwhile, the United Arab Emirates said it was ready to send ground troops into Syria as part of an international coalition fighting Islamic State, provided Washington took the lead. This echoed an offer made last week by its fellow Sunni Arab Gulf power Saudi Arabia.
Mr Erdogan said Turkey’s armed forces had the full authority to counter any threats to its national security, although senior officials were quoted saying Ankara did not intend to mount any unilateral incursion into Syria.
The United Nations last week suspended its long-awaited peace conference in Geneva until later this month, after Syrian forces launched their offensive and edged closer to retaking Aleppo.
Syrian foreign minister Walid Muallem said on Saturday that Damascus would send any invading forces home "in coffins".
