A aerial view shows a damaged mosque and surrounding buildings in the Al-Maysar neighbourhood of Aleppo, April 23, 2015.

Syria on Tuesday marked its fifth war anniversary, a grim milestone as peace talks were underway in Geneva with international powers aiming to end the conflict.

The U.N. refugee agency released a statement on the anniversary, saying international solidarity with its victims of the conflict is failing to match and reflect the scale and seriousness of the humanitarian tragedy.

“Syria is the biggest humanitarian and refugee crisis of our time, a continuing cause of suffering for millions which should be garnering a groundswell of support around the world,” said United Nations High Commissioner for Refugees Filippo Grandi.

Meanwhile, a fresh round of peace talks beginning on Monday were referred to as “Syria’s moment of truth.”

Saying there was no “plan B” but a return to war, U.N. mediator Staffan de Mistura asked to hear from all sides but said he would have no hesitation in calling in the big powers, led by the United States and Russia, if the talks get bogged down.

“If during these talks and in the next rounds we will see no notice of any willingness to negotiate… we will bring the issue back to those who have influence, and that is the Russian Federation, the USA… and to the Security Council,” he told a news conference.

The talks are the first to be held in more than two years and come amid an unprecedented cessation in hostilities sponsored by Washington and Moscow and accepted by President Bashar al-Assad’s government and most of his foes.

The truce, the first of its kind in a 5 year-old war that has killed 250,000 people, has sharply reduced the fighting over the past two weeks, giving rise to hope that this diplomatic initiative will succeed where all previous efforts failed. The cessation was agreed after de Mistura called off a previous attempt to convene talks last month.

On Monday night, President Vladimir Putin said “the main part” of Russian armed forces in Syria would start to withdraw.

Syria announced President Bashar al-Assad had agreed on the “reduction” of Russian forces in a telephone call with Putin. Western diplomats urged caution and the anti-Assad opposition expressed bafflement, with a spokesman saying “nobody knows what is in Putin’s mind.”

The Kremlin also said Putin and Syria leader Bashar al-Assad agreed on the withdrawal of Russian forces from Syria in a phone call.

Syria’s opposition on Monday, meanwhile, welcomed Russia’s announcement, saying a serious withdrawal would put pressure on Syrian authorities and give peace talks a positive impetus.

“If there is seriousness in implementing the withdrawal, it will give the talks a positive push,” said Salim al-Muslat, spokesman for the rebel High Negotiations Committee.

“If this is a serious step it will form a major element of pressure on the regime, because the Russian support prolonged the regime. Matters will change significantly as a result of that.”

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