* Llaima volcano's cone blocked with debris

* Experts fear build up of pressure, renewed explosions

* Intensity of eruption dies down

SANTIAGO, April 8 (Reuters) - The crater of Chile's erupting Llaima volcano, one of South America's most active, is blocked with debris that could prompt a pressure build-up and renewed explosions of lava, experts warned on Wednesday.

Llaima, which is in Chile's picturesque lake region about 435 miles (700 km) south of the capital Santiago, began spitting lava in a fresh bout of activity on Friday night.

It belched ash 4 miles (7 km) into the sky during the weekend, prompting the evacuation of 123 people from the surrounding area. Activity died down overnight and there is now a much smaller ash cloud but national emergency office ONEMI said the situation remained critical.

Chile's state mining and geology service, Sernageomin, said the cone of pyroclastic material, essentially molten rock, had built up and now jutted above the crater, extending the height of Llaima to 10,630 feet (3,240 meters) above sea level from around 10,397 feet (3,169 meters) previously.

"The main worry is that with pyroclastic material inside the main crater, the main conduit of the volcano -- the chimney -- is blocked to a certain extent and pressure could build up," said volcano expert Jorge Munoz, who works for Sernageomin.

"It would be much better if the crater were unobstructed, so it can release energy gradually and slowly," he added, saying the volcano's eruptive activity was very erratic and could quickly surge.

Llaima erupted fiercely on Jan. 1, 2008, and has expelled rock and ash sporadically since then.

Chile's chain of about 2,000 volcanoes is the world's second-largest after Indonesia. Some 50 to 60 are on record as having erupted, and 500 are potentially active.

Llaima was the second volcano to erupt in the past year.

The Chaiten volcano, 760 miles (1,200 km) south of Santiago in the Patagonia region, erupted last May for the first time in thousands of years, spewing ash, gas and molten rock and prompting the evacuation of thousands of people.

Ash from Chaiten soared 20 miles (32 km), swelled rivers and caused floods that damaged dozens of houses and destroyed much of the nearby town of the same name. The ash cloud from Chaiten coated towns in Argentina and was visible from space.

Chaiten erupted again in February, leading to the evacuation of residents who had rejected a government plan to abandon the town and rebuild it a few miles (km) away. (Reporting by Simon Gardner and Antonio de la Jara; Editing by Bill Trott)

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