49 deliberate grass fires in 12 hours in south Wales
A fire service said it has had to deal with 49 deliberate grass fires in just 12 hours.
Firefighters at South Wales Fire and Rescue Service have been called to various locations to tackle the blazes.
One included a mountain fire which spread to 1,000 tyres on farmland in Ynyshir, Rhondda Cynon Taff.
During the same period on Wednesday, the service received 500 calls and 73 fire engines have been used while crews are said to be "exhausted".
Most of the fires are now out and the one at Ynyshir is being monitored. Investigations will now focus on catching those responsible.
Arsonists have been accused of starting more than 400 grass fires in south Wales between 1-14 April, compared with about 120 in the same period last year.
They have cost the service about £795,000 and have left crews "stretched", say bosses.
The worst affected areas are Rhondda Cynon Taff with 145 fires and Caerphilly with 58.
The worst day saw 55 fires started in one day.
"The crews are pushed to breaking point," said Richard Hassett, fire station commander for Treorchy and Tonypandy in Rhondda Cynon Taff.
Nigel Williams, a spokesman for South Wales Fire and Rescue Service told BBC Radio Wales' Good Morning Wales: "It's a particular problem.
"There's potential to affect life and property. If you're considering such acts, please stop immediately."
In north Wales, there was a spate of deliberate fires near Blaenau Ffestiniog over two nights last week.