The United Nations human rights chief warned on Monday that thousands of people may have died of starvation during sieges affecting nearly half a million people in war-torn Syria.

The comments by Zeid Ra'ad al-Hussein came as the first major ceasefire in the five-year conflict entered its third day, and as the UN prepared to deploy lorries loaded with humanitarian aid into the country during the lull in fighting.

"The deliberate starvation of people is unequivocally forbidden as a weapon of warfare. By extension, so are sieges," said Hussein.

He added: "Thousands of people may have starved to death."

Al Jazeera's Jamal Elshayyal, reporting from the Turkish border town of Gaziantep, noted that US Secretary of State John Kerry had accused the Syrian government of using food as "a weapon of war".

"This war in recent months hasn't just been fought with weapons - it has also been fought through the use of food," he said. "The guns here haven't gone totally silent, so it's still dangerous for aid workers."

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The UN and its partner organisations were planning to start delivering aid to Syrians in several besieged areas previously cut off by the violence.

A UN spokesman told Al Jazeera that lorries bound for Mouadamiya in the southern outskirts of Damascus were loaded and were planning to move shortly.

Aid deliveries were also planned to arrive in the towns of Zabadani, Kefraya, Fouaa and Madaya by Wednesday.

The deliveries are part of humanitarian aid planned for 1.7 million people in hard-to-reach areas in the first quarter of 2016, Yacoub El Hillo, UN Resident Coordinator in Damascus, said in a statement on Sunday.

The UN estimates there are almost 500,000 people living under siege of a total of 4.6 million who are hard to reach with aid.

"It is the best opportunity that the Syrian people have had over the last five years for lasting peace and stability," El Hillo said.

"But we all know that without a meaningful political process and a political solution, both cessation of hostilities and entry of humanitarian assistance will not be enough to end the crisis in Syria."

Russian Foreign Minister Sergei Lavrov and John Kerry spoke on the phone on Sunday at the initiative of Moscow on the progress of the ceasefire, Russia's foreign ministry reported on Monday.

Syria's main opposition grouping, the High Negotiations Committee, on Sunday described the ceasefire as "positive", but lodged a formal complaint with the UN and foreign governments about breaches on the first day.

Its letter to Ban Ki-moon, the UN secretary-general, accused Bashar al-Assad's government and its allies of committing "24 violations with artillery shelling and five ground operations ... in 26 areas held by the moderate opposition".

The letter, signed by the committee's head Riad Hijab, also criticised Russia for conducting "26 air strikes on areas falling within the ceasefire".

It said that continued breaches of the ceasefire would make peace talks unattainable.

Syria's ally Russia has said that it has only targeted areas under the control of  al-Nusra Front, which is linked to al-Qaeda, and the Islamic State of Iraq and the Levant. Al-Nusra Front and ISIL are excluded from the terms of the international pact.

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Source: Al Jazeera and agencies