Four youths have been arrested over a series of fires in Sydney's southwest.

At least ten separate blazes around Macquarie Fields and Eagle Vale are believed to have been deliberately lit, with one of them threatened homes.

Two teens, aged 12 and 17, were arrested this afternoon after a grassfire broke out at Eagle Vale.

Police officers saw a fire on the side of the M5 at around 2:30pm.

Firefighters put out the blaze, while police spotted the two boys nearby.

The pair were arrested and taken to Macquarie Fields Police Station.

SUSPECTED FIREBUG: Police officers have arrested a Sydney teenager accused of arson offences. Photo: 7News

The state wide fire ban has now been extended for another day, as temperatures begin to soar again.