GENEVA (Reuters) - The United Nations is ready for a huge aid effort if the warring Syria parties stop fighting, but even then aid workers will proceed carefully and assess the safety of each delivery, a U.N. spokesman said on Wednesday.

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“We are now standing by, our warehouses are full of aid supplies, aid agencies are alerted, and are stocking all the goods in the warehouses waiting for the signal,” said Iyad Nasr, regional spokesman for the U.N. humanitarian aid office.

Syria’s war has killed over 250,000 people and left 4.5 million hard to reach with humanitarian aid, the U.N. says. Of those, about one in 10 is living under siege, cut off from any help.

A U.S.-Russian proposal for a limited “cessation of hostilities” from midnight on Saturday is intended to allow rapid and unhindered access for humanitarian agencies but it excludes Islamic State and Nusra Front fighters, and Syria’s opposition has yet to back the deal.

Aid has reached a handful of besieged towns in the past few days, and most recently into another -- Kafr Batna, on the outskirts of Damascus -- on Wednesday.

The government of President Bashar al-Assad bears responsibility for ensuring civilians receive aid and protection, but in practice all fighting groups need to consent to allow aid to reach specific areas, Nasr said.

When aid workers reached one government-besieged town, Madaya, in January, they found people starving to death or dying for lack of medical care, and others surviving on grass. Other towns could be in a similar state.

“We don’t know what is the exact situation until we are inside,” said Nasr.

Even if there is a “cessation of hostilities”, aid workers will not rush in recklessly.

“If we were talking about a total ceasefire we would be talking about a ceasefire all over the place. Until this happens we need to assess the security situation on the ground to ensure protection of civilians and humanitarian actors,” Nasr said.