The top human rights official of the United Nations called the Syrian government's siege of the capital's suburbs "an outrage" on Friday and said food and medical supplies must be allowed to reach civilians inside.

Residents of the Eastern Ghouta suburbs of Damascus are facing a "humanitarian emergency" as the price of food has skyrocketed in the region under siege, said Zeid Ra'ad al-Hussein in a statement published by his office.

Photos of children gaunt from hunger drew renewed attention this week to the Ghouta suburbs, one of the hubs of the 2011 uprising against President Bashar Assad's rule.

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The Syrian government routinely blocks the U.N. from delivering aid to areas opposed to its rule. The U.N. was last able to reach Eastern Ghouta a month ago, carrying supplies for only 25,000 people out of an estimated 350,000 in need.

The government has blocked three-quarters of the U.N.'s requests to deliver aid to besieged and hard-to-reach areas in Syria, according to the office of the U.N. High Commissioner for Human Rights.

The Syrian government's backers Russia and Iran, as well as Turkey, brokered a truce in May to allow sorely needed humanitarian aid to reach eastern Ghouta after nearly four years of siege and bombardment at the hands of government forces.

But the situation in the suburbs has deteriorated since government forces seized the Qaboun and Barzeh neighbourhoods in northeastern Damascus, also in May. The two neighbourhoods were the main points for smuggling supplies into the Ghouta region through tunnels.

Residents survive on smuggled goods, paying extortion prices to warlords and local traders who control the routes in and out of the enclave. Activists say two children have died of starvation in the last two months.

A kilogram (2.2 pounds) of sugar costs $12 in the district while a kilogram of rice costs nearly $5, according to local opposition activist Ahmad Khansour. The average family's monthly income is around $100, he said.

The government has at times during the war held over a million of its citizens under siege to force rebellious areas to accept President Bashar Assad's authority. Activists say the government is forcing its opponents to "kneel or starve," while the U.N. says starvation has become a weapon of war in Syria, saying it is a war crime.

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But it has proven brutally effective , and the government, acting largely with impunity, has found little reason to abandon its strategy. Pro-government forces tightly restrict the entry of food and medicine into the besieged areas as residents endure months, sometimes years, of relentless ground and air attacks. The Syrian government has depended on its key ally Russia to bomb these areas as well.

Government shelling killed six people in Eastern Ghouta on Thursday, according to Khansour. The Britain-based Syrian Observatory for Human Rights monitoring group said 11 people were killed.

The Washington-based Siege Watch monitoring group says more than 800,000 people in Syria are still trapped in sieges, most set by the government and its allies.

The Eastern Ghouta suburbs were one of the first areas in the country to organize self-governing councils called Local Coordination Committees as an alternative to Assad rule, but their efforts have been largely erased by the scale of the violence that has washed over the country since 2011.

More than 400,000 people have been killed and half the country's population has been displaced in the civil war that has drawn armies and militia members from around the globe.