Up to 1.5 million Syrians are in urgent need of food, water, shelter and sanitation as parts of the embattled country descend into civil war, the head of the Red Cross said yesterday.

And Kofi Annan, the joint UN-Arab League envoy, told the UN Security Council that such a war might be inevitable if his increasingly fragile peace plan collapses.

Jakob Kellenberger, president of the International Committee of the Red Cross, said tens of thousands of people were taking shelter in public buildings or other people's homes, and the Red Cross and Syrian Arab Red Crescent were feeding about 100,000 "particularly vulnerable" Syrians. He said fighting had been so intense that the conflict in some parts of Syria qualified as a localised civil war.

Syrians are also grappling with fuel supply shortages after western sanctions prompted most European oil firms to halt trade with Syria. Industry sources said Syria faced a halt in imports of diesel needed to power heavy vehicles, including army tanks, as shipments from Russia and other sources has dried up.

The halt in Russian supplies could exacerbate the pain already felt by many struggling to make a living with limited access to fuel and power.

"We can't farm because they've cut off the electricity and they haven't been distributing fuel. So we can't use our farming equipment, for example. We've lost our livelihood", said Melhem, a farmer from Hasakeh.

Mr Annan, meanwhile, said the UN's ceasefire observer mission was "possibly the only remaining chance to stabilise the situation".

"There is a profound concern that the country could descend into full civil war," he said after briefing the Security Council. "That would be frightening. We cannot allow that to happen.

"We are trying to push it as hard as we can, but we may well conclude that the plan doesn't work and a different tack has to be taken."

The envoy warned that civil war "will have an impact in the whole region. That is why we should be all concerned for Syria and Syrians and for … geopolitical reason, we should all be concerned."

Mr Annan's six-point calls for a cessation of all violence and withdrawal of Syrian government heavy weapons and forces from populated areas; the start of a political dialogue leading to free national elections; freedom of assembly and protest; freeing of political prisoners; access for humanitarian aid, and access for news media.

"All aspects of the six points must be implemented for us to create a conducive environment to move on to dialogue," Mr Annan said. "It is extremely difficult if the current conditions persist."

While there have been "some decrease in military activities there are still serious violations in the cessation of violence that was agreed," Mr Annan said. "The level of violence is unacceptable. Government troops and armour are still present though in smaller formations."

While Mr Annan said the government had the primary responsibility to stop the killing, he blamed opposition groups too. "We have also seen attacks against government troops and installations as well as spate of bombings that are very worrying to the security of the population."

He called on governments with influence on both sides to increase the pressure to stop the fighting.

The unarmed UN monitors and their civilian staff are supposed to help to implement the plan, principally reporting on ceasefire violations. The 60 observers already deployed "have had a calming effect and have sometimes been able to get the forces involved to the right thing," Mr Annan said. "There has been less shelling."

Mr Annan said it might be difficult for fighters on both sides to lay down arms. "But we all remember that they did on the 12th of April", the first day of the ceasefire. "If you can do it for one day, why not a week, a month," he said.

"Give the people of Syria a break," he implored. "Why must they put up with this trauma?"

In Rome yesterday, the Turkish prime minister Recep Tayyip Erdogan called for a UN-backed mission of 3,000 observers, in part to stem the flow of Syrian refugees into Turkey.

"I've lost hope that we can find a desired solution," Mr Erdogan said. "What can 50 observers do? We need perhaps 3,000 observers in a large mission."

Mr Erdogan said 23,000 Syrian refugees have arrived in Turkey so far.

Gen Robert Mood of Norway, who is heading the UN ceasefire monitoring mission, said last week that even 10,000 observers could not end the killing if the parties wanted to continue fighting.

The Security Council has so far authorised 300 monitors for 90 days. Only 60 are on the ground, with the rest expected by the end of May.