ATHENS, Greece (AP) — Hundreds of firefighters backed by water-dropping planes and helicopters struggled for a third day Thursday to tackle a wildfire burning through a pine forest on the Greek island of Evia.

Nearly 400 firefighters, 11 helicopters and 13 planes — including two from Italy and one from Spain sent in following Greece's request for help — fought the blaze which broke out hours before dawn on Tuesday and has ravaged a nature reserve.

The fire department said Thursday evening that the wildfire was "developing." The most active parts of the blaze were far from inhabited areas, burning through dense pine forest in canyons hard to reach by land.

Residents gradually returned to four villages that had been evacuated on the first day, but were on standby should they be required to leave again.

A state of emergency has been declared in the area, and Greece requested firefighting assistance from other European countries. A volunteer firefighter who suffered burns underwent surgery on Thursday, and remained hospitalized.

Another 39 forest fires broke out in the 24 hours from Wednesday evening to Thursday evening across the country, the fire department said, with around 1,000 firefighters, volunteers and the military tackling the blazes. Most were brought under control in their initial stages, it added.

Separately, two men — a 58-year-old and a 52-year-old — were arrested on Wednesday and Thursday on suspicion of arson in two separate forest fire cases in southern Greece.

Wildfires are common in Greece during the hot, dry summer months. Authorities have repeatedly warned the public not to engage in outdoor activities that could cause fires, such as welding work, burning weeds or lighting campfires and barbecues. Parks and forest areas are closed at times of high fire risk, and daily bulletins are issued on the level of fire risk around the country. Causing a forest fire is a criminal offence in Greece.

Last summer, more than 100 people died when a fast-moving forest fire broke out in a seaside area northeast of Athens and raged through the vacation-home settlement of Mati. The blaze trapped many in homes, on narrow paths and in their cars as they attempted to flee, while others drowned while trying to swim away from the heat and choking smoke engulfing beaches.