Beirut (AFP) - Aid groups pressed on with talks to evacuate hundreds of people, many starving, from a besieged Syrian town, as the UN warned more would die unless blockades are lifted.

Elsewhere in Syria, forces loyal to President Bashar al-Assad scored a landmark victory against rebels by capturing the key town of Salma, in the country's northwest.

The United Nations, the International Committee of the Red Cross, and the Syrian Arab Red Crescent held talks on Tuesday to evacuate around 400 people from the starvation-struck town of Madaya.

More than two dozen people have reportedly starved to death there, crippled by a six-month government siege that has made even bread and water hard to find.

The UN humanitarian coordinator for Syria, Yacoub El Hillo, warned that "many more will die" unless government forces and rebels lift sieges of towns across the country.

El Hillo, who toured Madaya where he said he saw "severely malnourished people", described the blockades as the "tactic of war" being used by all sides in the nearly five-year conflict.

"It must stop," he said. "Many more will die if the world does not move faster" to end the sieges, which he said were chiefly to blame for the suffering.

On Monday, the first aid trucks in about four months entered Madaya, delivering desperately needed food and medicine.

At least one more delivery is expected in the coming days, a UN official said.

But hundreds of residents remain in need of urgent care, and aid groups are working on their evacuation, said ICRC spokesman Pawel Krzysiek.

"It's a very complicated process that needs permission to realise this humanitarian operation. We are in negotiations with all parties," he told AFP, adding that it could take some time.

- 'No comparison' -

The level of suffering in Madaya has no precedent in Syria's war, the UN refugee agency's representative in Syria said.

"There is no comparison in what we saw in Madaya," Sajjad Malik told reporters in Geneva.

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"There are people in Madaya, but no life. What we saw in Madaya should not happen in this century," said Malik, who travelled to Madaya on Monday.

"We want to make sure the siege is lifted."

The town is part of a six-month UN-brokered truce that also includes the nearby rebel-held town of Zabadani, and Fuaa and Kafraya, two government-controlled towns in Syria's northwest.

The agreement, reached in September, foresaw an end to hostilities in the four towns in exchange for humanitarian aid and some evacuations.

The situation in Fuaa and Kafraya also "remains very concerning," with little drinking water, food, or electricity, said Krzysiek.

Syria's regime has championed localised ceasefires as a way to end the conflict, which has killed 260,000 people.

But with the war approaching its sixth year, UN agencies on Tuesday appealed for $7.73 billion in funding to help 22.5 million people affected by the conflict.

- Major regime victory -

But government forces have also ramped up ground offensives against rebels across the country, with air support from Russia.

On Tuesday, the army and National Defence Forces militia took full control from rebels of the strategic town of Salma, in the northwestern province of Latakia.

State television said pro-government forces also seized surrounding hilltops and were combing them for landmines and explosive devices.

The army's command described Salma as "the main operational centre for terrorist groups" in the area and its recapture was an "important accomplishment".

Seizing Salma -- the main bastion of opposition forces since 2012 -- is a major boost for Syria's beleaguered army, which had been mostly locked in a stalemate with rebel factions in the province.

Regime forces have fought fierce battles in recent months to retake rebel-controlled areas with help from Iran-backed Hezbollah fighters and from Russian air strikes.

The Syrian Observatory for Human Rights said Russia conducted more than 120 air strikes over 48 hours in support of the army's Salma offensive.

And on Tuesday, suspected Russian strikes killed 57 civilians, including children and paramedics, in Idlib province, adjacent to Latakia, and in Aleppo in the north, the Observatory said.

Rights groups have condemned Russia for killing civilians in its air war, but Moscow insists it is fighting extremist groups.

Meanwhile the UN said its envoy to Syria would meet on Wednesday with ambassadors from the five permanent members of the UN Security Council.

Staffan de Mistura is trying to shore up support for planned peace talks later this month between Damascus and the opposition.