(CNN) People in Aleppo are hungry and terrified as the Syrian army tightens its grip on the rebel-held eastern section of the city, resident and photojournalist Karam Al Masri told CNN.

Food and water have become scarce, he said.

"The situation now is really bad. There is not enough food in the city, not enough bread. The residents are very scared and hungry."

Many markets are closed and people stand in long lines for food, with each family receiving six loaves of bread every two days, Masri said.

"There is a shortage of all the basic needs," he added.

Rebels urged to surrender

The Syrian army said Wednesday its troops and supporters have surrounded Aleppo and cut off "all supply lines and corridors" to the rebel-held neighborhoods, state-run media reported.

The army called on the rebels -- everyone "bearing arms" -- to surrender their weapons and either leave or remain in the city, according to SANA

Residents received texts from the regime telling them to leave, CNN's Nick Paton Walsh reported from Beirut.

Masri said it would be difficult for residents to get out of the city, even if they wanted to.

"The eastern parts of the city are completely cut off by the regime forces," he said. "There is no escape. ... All the roads are cut off and the city is besieged."

He said people fear the Syrian army will kill everybody if they advance into the city.

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"But that is not likely to happen," he said. "The likely scenario is that the regime won't get inside the city. Urban fighting inside Aleppo is very difficult. Instead, they will starve the residents and force surrender, just like they did in Homs."

The United Nations said it is deeply concerned about a potential humanitarian disaster. Aid workers report the only road in or out is effectively cut off and they cannot evacuate patients along it.

The UK-based Syrian Observatory for Human Rights said barrel bombs dropped by Syrian helicopters killed at least five civilians Tuesday in the Mashhad district of Aleppo.

In another incident, a child was killed when a residential building collapsed following a barrel bomb attack in the Salah al-Deen district, SOHR said.

UNICEF says hospitals struck

The siege has been especially rough on medical personnel , who face multiple challenges in operating hospitals. Losing access to the road out of Aleppo only makes it more difficult for them to obtain medical supplies.

UNICEF issued a statement saying four hospitals in eastern Aleppo -- al-Dakkak, al-Bayan, al-Hakim and al-Zahraa -- and a blood blank were struck in recent days, disrupting key life-saving health services.

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"Health facilities in Syria are being attacked with alarming ferocity," the statement said. "According to([World Health Organization) estimates, since the beginning of the year, there have been more than forty attacks on health facilities in Syria. It is estimated that nearly 60% of public hospitals in the country have closed or are only partially functional.

"Attacks on health facilities are a blatant violation of international humanitarian law and can amount to war crimes. Health facilities must never be attacked or damaged, and health workers should be allowed to provide medical treatment and services to all people in need wherever they are inside Syria."

Aleppo has been a major battlefield in Syria since 2011, with fierce fighting between rebel groups and regime forces.

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According to the SOHR, Aleppo has seen many of its neighborhoods come under fire for 80 consecutive days, with more than 6,000 people -- mainly civilians -- killed or injured.

SOHR said rebel-held areas in the east of the city have come under sustained attack by regime artillery and airstrikes, while regime-controlled areas in western neighborhoods have been shelled by rebels.