LISBON (Reuters) - Clouds and cooler weather on Thursday helped snuff out forest fires that have ravaged central Portugal, but officials braced for further outbreaks as forecasters said temperatures would heat up at the weekend. 

For the first time in many days, the national fire service reported no blazes. But 900 firefighters, backed by troops, remained on alert, it said. 

Temperatures were expected to drop until Friday, with much of Portugal under cloudy skies, the weather service said. In the central city of Coimbra, close to the worst fires, the thermometer was expected to reach only a relatively modest 25 Celsius (77 Fahrenheit) on Friday. 

A house is seen after a forest fire in Miranda Do Corvo, near Coimbra in central Portugal August 24, 2005. Clouds and cooler weather on Thursday helped snuff out forest fires that have ravaged central Portugal, but officials braced for further outbreaks as forecasters said temperatures would heat up at the weekend. (REUTERS/Nacho Doce)

The cooler weather brought relief to 4,200 volunteer firefighters who "have been stretched to the limits of their resistance", Interior Minister Antonio Costa said late on Wednesday. 

But Costa said an expected upturn in temperatures and clearing skies starting on Saturday could bring renewed fires. 

Portugal is enduring its worst drought on record. The National Forest Fire Authority estimates that more than 180,000 hectares (450,000 acres) of woodland have burned so far this year, already the second-highest annual figure since 1980. 

Since Sunday, aircraft from France, Germany, Italy, the Netherlands and Spain have helped fight the fires that have killed at least 14 people, according to the Lusa news agency.