Gritters have been putting in overtime this week across the UK
The gritting salt keeping Britain's roads clear of snow and ice is running out amid a warning from the AA that some roads are becoming "death traps".
Many councils in England and Wales are having to ration supplies and have stopped using salt on minor roads.
The AA wants the government to lead a co-ordinated response to the crisis.
The Highways Agency, which grits all motorways and some A-roads, said it was doing what it could to assist councils struggling to keep roads gritted.
The head of its National Traffic Control Centre, Steve Crosthwaite, said: "We've already written to all local authorities, saying we will take any requests and have a look at them in each merit.
Running on empty
"Obviously that has to be compared to what our local situation's like, to make sure that we've got sufficient [salt] to keep the motorways and major trunk roads running. But if we have got surplus we will help out local authorities and indeed we have been doing that in some places."
The Highways Agency is putting 25,000 tonnes of salt on main roads a day, but producers can only deliver 30,000 tonnes a week.
Made from rock salt carved from underground mines
Mine deposits formed as ancient bodies of salt water dried up
Usually brown in colour and resembles gravel
The AA said in a statement that several local authorities, who grit everything from the smaller A-roads, all B-roads and minor roads and streets, had either "run out of salt or have such low stock levels that very few roads are being gritted."
It said the worst areas appeared to be Wiltshire, Hertfordshire, Surrey, Derbyshire and parts of Wales.
AA's president Edmund King added: "The harshest winter for almost two decades has left some highway authorities running on empty as regards salt stocks. Many are desperate to re-stock their road salt but supply chain pressures from mine to highway depot looks like resulting in some areas running dry.
"The government should step in to assess the situation and ensure that salt stocks are maintained in the places at immediate risk from snow and ice over the coming days."
He added: "This is a very serious situation, with some roads becoming death traps."
RAC motoring strategist Adrian Tink added: "The number of roads that haven't been gritted is a big concern and has a serious impact on driver safety.
"With UK motorists giving the government £45 billion a year in taxes, they will feel pretty annoyed there isn't enough cash to keep all the road networks moving."
Bristol City Council has admitted that grit supplies are starting to run low. It has just 200 tons left, having used 70 on Wednesday night.
Warwickshire County Council has confirmed it is rationing gritting, while in Wiltshire only main routes are being gritted.
We cannot expect to keep every side road right across the country open given the very serious weather conditions
Geoff Hoon, transport secretary
There, smaller roads have become extremely icy in patches and some have been blocked by jack-knifed lorries or double-deckers stuck in the road.
Gloucestershire County Council said it had halved its salting operation to preserve stocks as it tries to hunt around for alternative supplies.
In Surrey, sand is being used to treat secondary roads and pavements to conserve supplies of salt for main roads. The county council said sand was a "highly effective way of providing traction" and would be used on pavements next to main roads, in town centres and around some secondary schools.
Transport secretary Geoff Hoon said: "We are taking action across the country to ensure that in those areas where salt is in short supply we are getting round to those areas to make sure it's available.
"We cannot expect to keep every side road right across the country open given the very serious weather conditions we have faced over the last few days.
"Clearly, if the weather continues to be difficult as it has been in the past few days then we are putting down on to the roads far more salt than can physically be produced by the suppliers."
He added that it was also "perfectly possible" that the country could have more snow ploughs at its disposal.
"But we would all then have to consider the cost of that - do we want to maintain large numbers of snow ploughs in storage for an event that occurs only every 18 years?"
The deputy chairman of the Local Government Association, Richard Kemp, said there had been an unprecedented demand on gritting salt because of the severity of the weather.
He told the BBC News channel: "Basically, we have a storage capacity. You can't just dump the grit on a piece of waste space.
"It all has to be under cover, and we're going well beyond the worst contingency plans of local government. We plan for normal, we plan for abnormal, what we can't plan for is the exceptional that we're getting at the moment."
Britain's biggest salt supplier, Cheshire-based Salt Union, said staff were working round the clock but still could not meet demand.
The firm said: "We have been operating 24 hours a day, seven days a week since the beginning of January and are extracting 30,000 tonnes per week but the unexpected and unusual weather means that, even working at this level, demand is outstripping supply."
The biggest supplier in the West Midlands, DA Baldwin and Son in Wolverhampton, has completely run out of supplies, while a Teesside mine, Cleveland Potash in Saltburn, is importing 40,000 tonnes of salt from a sister company in Spain.
The British Salt depot in Middlewich, Cheshire, has 100,000 tonnes of low-grade salt available, a stockpile that has accumulated over the last 15 years as a by-product in the manufacture of table salt.
A queue of 200 lorries was forming on Thursday to collect much-needed supplies.