New Zealand is leading urgent talks at the United Nations on a desperate humanitarian crisis in the Syrian town of Madaya, where besieged residents are reported to be dying of starvation.

Along with Spain, New Zealand today called for the United Nations Security Council (UNSC) to hold consultation behind closed doors on the situation in Madaya and other besieged areas in the country.

"The tactic of siege and starvation is one of the most appalling characteristics of the Syrian conflict," New Zealand's permanent representative to the United Nations Gerard van Bohemen said.

"It's encouraging that a convoy of humanitarian supplies has arrived in Madaya today, but this is just the start. We need unimpeded and sustained humanitarian access to all those in need in Syria."

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In a media conference, Spain's ambassador Roman Oyarzun Marchesi said the council had been told that 400 people in Madaya were in a "very critical situation" and needed to be evacuated tonight (US time).

If they were not evacuated immediately, "the situation will be more than dramatic tomorrow", he said.

Mr Oyarzun said the humanitarian concerns were not limited to the town of Madaya, which is situated in the southwest of the country near the Lebanese border.

"I think that you are all aware that we are talking about 400,000 people living under besieged areas in Syria.

"It's not only about Madaya, in Madaya we have 42,000 people. It's also about many other besieged areas by Daesh (Isis) and other groups that we have to take care of."

Mr van Bohemen also responded to claims from Syrian officials that there was no starvation in Madaya and the crisis had been fabricated by "hostile governments" and media.

He said there was "definitely a situation" in the township and this had been witnessed by the aid convoy when it arrived today.