There are fears the flames could spread to an observatory housing the world's largest telescope.
The Roque de Los Muchachos Observatory has been operating for more than 20 years, and a new telescope was inaugurated last month.
Fresh teams of firefighters have been brought in from other parts of the Canary Islands, and Spanish Prime Minister Jose Luis Rodriguez Zapatero has also arrived in La Palma to inspect efforts to tackle the fire.
He said an "appropriate decree regarding aid" would be approved at the next cabinet meeting on 13 August.
The fire is thought to have been caused by a stray firework let off during local fiestas.
Some progress has been made in combating the fires after temperatures eased slightly, but a fire close to the town of Mazo - a popular tourist destination - remains out of control.
TV pictures showed flames metres away from villages in the Tigalate area, where a major containment effort is under way.
Water-dumping planes and helicopters managed to keep the flames away from the homes, but residents were forced to evacuate.
British holidaymaker Cath Watkin said the fire came dangerously close to the hotel where she was staying.
"We experienced extremely hot, searing winds, and then woke up in the early hours of Saturday morning to very, very thick, choking smoke from the fires, extremely hot outside," she said.
"The fire was actually very close: the hotel is actually at the bottom of the mountain, the fires were raging on the mountain behind the hotel."
Dozens of tourists have been unable to catch flights home because of road closures.
La Palma is a small, relatively undeveloped island, with a resident population of about 85,000.
It is dominated by a dormant volcano, and is renowned as one of the greenest of the Canary Islands. But its lush vegetation has been seared by the high temperatures of recent days.
Smaller fires have broken out on other Canary Islands, including Tenerife and Gran Canaria, but are expected to be brought under control soon.
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