(CNN) Shortly after aid trucks finally reached Madaya, the situation turned out to be even more dire than expected.

Hundreds of civilians in the besieged Syrian city are on the brink of death, said Stephen O'Brien, U.N. under-secretary-general for humanitarian affairs,

"... we found some 400 people who must be evacuated IMMEDIATELY for medical treatment or face dying #Syria," O'Brien tweeted.

I told SecCo that today in #Madaya , we found some 400 people who must be evacuated IMMEDIATELY for medical treatment or face dying #Syria

Madaya has been ravaged by starvation in the Syrian civil war. The rebel-held city has been choked off by the regime and landmines.

On Monday, the city of 40,000 received its first shipment of foreign aid since October. The sight of food trucks brought starving residents to tears, a U.N. source told CNN.

The U.N. Refugee Agency said 49 vehicles were delivering aid to Madaya.

According to SANA, Syria's state news agency, 65 trucks loaded with aid supplies entered Madaya and two other besieged towns, Foua and Kefraya.

"It's heartbreaking to see so many hungry people," said Sajjad Malik, the UNHCR representative in Syria. "It's cold and raining, but there is excitement because we are here with some food and blankets."

Shocking images of starving residents of Madaya have garnered international attention.

Photos: Madaya: Activists post images of what they say are starving locals Photos: Madaya: Activists post images of what they say are starving locals Demonstrators in Idlib, Syria, hold banners in a protest for civilians in besieged Madaya on Tuesday, January 5. Hide Caption 1 of 5 Photos: Madaya: Activists post images of what they say are starving locals Hide Caption 2 of 5 Photos: Madaya: Activists post images of what they say are starving locals One activist in the Syrian town of Madaya posted this image on social media of Suleiman Abdul-Karim Fares, 40, who had died from starvation on January 4, 2016. CNN cannot independently verify the image or information. Hide Caption 3 of 5 Photos: Madaya: Activists post images of what they say are starving locals One activist in the Syrian town of Madaya posted this image on social media of Abdel Karim Jawad, 60, who had died on January 2, 2016 due to starvation and lack of access to medication. CNN cannot independently verify the image or information. Hide Caption 4 of 5 Photos: Madaya: Activists post images of what they say are starving locals This image of a hungry child was posted on social media by an activist in the Syrian town of Madaya. CNN cannot independently verify the image. Hide Caption 5 of 5

Graphic images of death and starvation coming out of Madaya have not been independently confirmed by aid groups or CNN.

But the United Nations said last week that it had received credible reports of people dying of starvation and that the Syrian government had agreed to allow aid convoys into Madaya, Foua and Kefraya.

The situation has been so dire that a doctor told CNN he has nothing to give his patients except sugar or salt water. In one video posted by Syrian activists, a skeletal boy -- his ribs protruding -- says he hasn't eaten a full meal in seven days.

The convoy for Madaya came from the U.N. World Food Programme, International Red Cross and Syrian Arab Red Crescent. It should sustain 40,000 people for a month, WFP spokeswoman Abeer Etefa said.

The U.N. source told CNN that an equivalent amount of aid would also enter the regime-loyal towns of Foua and Kefraya, in the northern province of Idlib, which were enduring a similar plight while under siege by rebels.

Syrian government denies starvation

Syrian Ambassador to the U.N. Bashar al-Ja'afari denied that anyone is starving in Madaya, calling the images of starving people "fabrications." He said his government had appealed for humanitarian assistance weeks ago.

A photo taken by a local councilman shows cars belonging to aid agencies arriving in Madaya on Monday.

"The problem is the terrorists are stealing the humanitarian assistance from the Syrian Red Crescent as well as from the United Nations," al-Ja'afari said.

He also denied the Syrian government is using starvation as a tool of war, which is generally considered a war crime.

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"The Syrian government did not stop any convoys of humanitarian assistance," he said. "On the contrary: We sent plenty of convoys and we asked the U.N. to send more.

But O'Brien, the U.N. official, refuted that claim.

"I can tell you that we have had confirmation of extreme malnourishment of a number of people across all ages," he said.

He told reporters that the U.N. has received "reports of people who are either starving or indeed have starved and died."

More than 250,000 Syrians -- largely civilians -- have been killed in the civil war , according to the United Nations.

Unbearable costs and landmines

Even though Madaya is less than 50 kilometers (31 miles) from the capital city of Damascus, the cost of food has crippled the city.

CNN Map

For example, in Damascus, flour costs 79 cents a kilogram. But in Madaya, a kilo of flour costs $120, and a kilo of rice costs $150.

In the capital, milk costs $1.06 a liter. But in Madaya, the price soars to $300 a liter.

Then there's the problem of landmines, which have made smuggling food into the city extremely dangerous, Dr. Khaled Mohammed said.

Mohammed, who works at a field hospital in Madaya, said he gets about 250 cases of starvation a day. He said the hospital has seen at least 55 deaths from starvation.

On Sunday, he said five people died in the past 48 hours, including a 9-year-old child.

And when a child dies, the doctor said, it's likely his or her siblings will die soon, too. So families go door to door, urgently trying to gather what they can to feed and save the siblings.