11-year-old arrested as tough action is promised on grass fires
Children as young as 11 are among those who have been arrested after grass fires ravaged the countryside.
South Wales Police said it will take a tough stance on "naive and reckless actions" before someone is killed.
Fire crews in south Wales have dealt with 623 deliberately-set grass fires since April 1 and received more than 900 calls on Saturday alone.
Firefighters were also called to a six-hectare (14.8-acre) fire above the Gaer, Newport, at 21:00 BST Monday.
Deliberately set mountain fires at Machen, Caerphilly, and Clydach Vale, Rhondda, also continued to burn on Monday night.
They were left to burn within controlled boundaries, such as roads, with some terrain described as "treacherous" by the fire service.
They are being managed and monitored and crews will return if fires threaten property, forestry or become dangerous.
A helicopter was used to help tackle a mountain fire more than 1km wide in Rhondda.
It scooped water from a reservoir to drop on the blaze at Clydach Vale, Tonypandy.
Firefighters also tackled another huge blaze at Machen, Caerphilly, with 150 hectares (370 acres) of grass and bracken burning.
Four youths between 12 and 14 were arrested in Maerdy, Rhondda, on Sunday and bailed pending further inquiries while an 11-year-old was arrested in Taff.
Following a weekend that included calls to 156 grass fires, 137 started deliberately, South Wales Police Deputy Chief Constable Matt Jukes promised tough action.
He visited Porth in the Rhondda valley, an area that has suffered more incidents than any other in south Wales since March.
DCC Jukes said: "I can see that it is only a matter of time before somebody gets seriously injured or even killed.
"What is most concerning is that we believe that many of these grass fires are being caused by children."
He urged parents and teachers to look out for signs, such as clothes smelling of smoke and youngsters possessing matches or accelerants.
"People from a very young age risk spoiling their prospects forever, through one act of stupidity," DCC Jukes added.
Mid and West Wales Fire Service say they dealt with eight grass fires on Monday and a total of 50 over the weekend.
A spokeswoman said on Monday there were six hectares (15 acres) alight in Crymych and another large fire at Penycae, Port Talbot.