KILIS, Turkey - Turkey must open its doors to the tens of thousands of Syrians who have massed at the border after fleeing violence, the U.N. demanded Tuesday, as an aide group said tents on the Syrian side are overcrowded and food in short supply.

Turkey, already home to 2.5 million Syrian refugees, insists it has an open-door policy toward Syrians escaping conflict but has still kept the key Bab al-Salameh border crossing closed for days. Government officials say Turkey will provide assistance to the displaced Syrians within their own borders "as much as possible" and would allow them in "when necessary."

On Sunday, Turkish Deputy Prime Minister Numan Kurtulmus said: "Turkey has reached the end of its capacity to absorb (refugees), vbut 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."

Get Breaking News Delivered to Your Inbox

UNHCR spokesman William Spindler asked Turkey on Tuesday to open the border to "all civilians who are fleeing danger and seeking international protection as they have done since the start of this crisis."

Spindler also urged other nations to shoulder more of the refugee burden and to work to end the conflict.

"The answer to this crisis is for the peace process to continue in Syria and for the conflict to be solved," Spindler said.

The war in Syria against President Bashar Assad's government has killed over 250,000 people and forced millions to flee their homes since it began in 2011. In recent days, a Russian-backed Syrian government offensive around the country's largest city Aleppo has sent an estimated 75,000 Syrians fleeing toward the border with Turkey, according to Abdulsalam al-Shareef, a consultant for the Turkish charity group Humanitarian Relief Foundation, or IHH.

The group, which has been distributing food, medicines and blankets at the border, said Tuesday it was expanding an existing displaced persons camp a few miles away from Bab al-Salameh and was "surveying" sites for possible new camps.

"Today the situation got worse because yesterday night and (this) morning, Russian jets started bombing two big cities, Tel Rifaat and Marea," he said. "It is really a big disaster."

Although some humanitarian aid has arrived, there is still quite a large need for basic humanitarian assistance because of the sheer scale of the situation, said Dalia al-Awqati, north Syria director for the Mercy Corps aid group.

"The ability to provide individual shelter for new arrivals is definitely being challenged," she said. "The tents are definitely overcrowded and the food is overstretched."

Al-Awqati added the displaced Syrians are not likely to return because their homes "are no longer accessible."

On Tuesday, the U.N. also warned that hundreds of thousands of others in Aleppo could soon be cut off from humanitarian aid amid blistering Syrian and Russian airstrikes.

The U.N. humanitarian office OCHA said 300,000 people could be cut off from aid if the Syrian government and allied forces encircle the city of Aleppo and deprive those fleeing from their last way out. OCHA said local leaders believe up to 150,000 people could try to flee to nearby Afrin and the surrounding countryside.

Turkish Foreign Minister Mevlut Cavusoglu estimated that up to a million more people could flee if the onslaught continues.

The country's Prime Minister Ahmet Davutoglu urged the world to speak out against Russia for "mercilessly bombing civilian targets" in Syria. He predicted that Russia would eventually retreat from Syria in "embarrassment" - in a way similar to the Soviet retreat from Afghanistan.

"Those ... who turned Syria into a bloodbath will certainly pay for it one day," Davutoglu said.

Russia has insisted its warplanes target the Islamic State and other extremist groups and has firmly denied that they hit civilian areas.

Several aid trucks could be seen moving Tuesday through Turkey's Oncupinar border crossing, opposite the Bab al-Salameh gate in Syria, which remained shut to refugees for the fifth straight day.

One of the few Syrians who managed to cross into Turkey from Oncupinar recently was six-year-old Aya al-Sharqawi - who was wounded in Russian airstrikes 10 days ago at her hometown of Tel Rifaat - and her uncle, Abdelrahman al-Sharqawi.

"I went to buy (biscuits)," the girl said from her hospital bed at the Kilis hospital. "When the plane came I started running. I was near the entrance of my sister's house when the plane came and dropped a bomb."

Her uncle said the girl was first taken to a field hospital in Tel Rifaat, which then sent her to Turkey for further treatment.

Also being treated at the Kilis hospital was Ejnad Akkad, a fighter for the opposition Free Syrian Army, who was wounded by a mine planted by the Islamic State of Iraq and Syria (ISIS) near the border with Turkey.

"The Russian planes have decimated people. The (improvised explosive devices) and mines belonging to (ISIS) have also slaughtered people," he said. "The (Syrian) regime is advancing only thanks to the Russian warplanes and Shiite militias."

At a joint news conference with his Hungarian counterpart in Budapest, Cavusoglu said Turkey was admitting refugees "in a controlled manner" and that 10,000 Syrians had been allowed in recently. He did not elaborate, and it was not clear if he was referring to the thousands of Syrian Turkmens who were admitted into Turkey earlier this month.

The U.N.'s World Food Program said Tuesday it has begun shipping lentils, pasta, beans, vegetable oil, wheat flour, sugar and other items to the town of Azaz northwest of Aleppo, near the border with Turkey, and hopes to continue in coming days.

"We are extremely concerned, as access and supply routes from the north to eastern Aleppo city and surrounding areas are now cut off," said Jakob Kern, the WFP country director for Syria. "But we are making every effort to get enough food in place for all those in need, bringing it in through the remaining open border crossing point from Turkey."