GENEVA (Reuters) - The United Nations still has a million questions about a Syria deal struck last week by Russia, Turkey and Iran, with aid convoys almost totally stalled despite a reported reduction in the fighting, a U.N. aid official said on Thursday.

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“Russia and Turkey and Iran explained to us today and yesterday ... that they will work very openly, proactively, with United Nations and humanitarian partners to implement this agreement,” U.N. Syria humanitarian adviser Jan Egeland told reporters.

“We do have a million questions and concerns but I think we don’t have the luxury that some have, of this distant cynicism, and saying it will fail. We need this to succeed.”

The three-country deal on de-escalation zones was signed last week in the Kazakh capital Astana, with a goal of resolving operational issues, such as how to police the zones, within two weeks and of mapping them out by June 4.

U.N. Syria envoy Staffan de Mistura said he was convening what he called “rather business-like, rather short” peace talks in Geneva from May 16-19 to take advantage of the momentum. The political weight of the signatories and the staggered timing of the deal gave it a strong chance of working.

The alternative would be “another 10 Aleppos”, he said, referring to Syria’s second city which fell to forces loyal to President Bashar al-Assad in December after years of fighting.

A key aspect of the Astana deal was that it was an “interim” arrangement to address urgent issues, not a permanent partition of Syria, he said.

De Mistura said the Astana talks had also made rapid progress on agreements covering prisoner releases and de-mining, and both were almost complete.

Egeland said he could point to one concrete result from Astana: the reported reduction in fighting and aerial attacks. But aid convoys were only being allowed in at the rate of one per week, with no permission letters coming from the government.

Although some recent local surrenders meant the number of people who were hard to reach with aid had fallen by 10 percent to 4.5 million, a further 625,000 were besieged - 80 percent of them by forces loyal to Assad, he said.