The alert level of the volcano on Sulawesi Island was again raised to the highest level by the Indonesian geological service, after sensors picked up increasing volcanic activity. The agency urged residents and climbers to stay at least 6 kilometers (3.7 miles) from the volcano's crater.

The 725-metre (2,378-foot) volcano in North Sulawesi province is about 95 kilometers (59 miles) northeast of Sam Ratulangi International Airport in Manado, the provincial capital.

The airport was closed on Tuesday morning due to reduced visibility and the dangers posed to aircraft engines by ash, said Ambar Suryoko, head of the regional airport authority.

Ash, grit and rock fell from the sky in towns and cities across the region, including Manado, a city with more than 430,000 people where motorists had to switch on their headlights during daytime.

“It was dark with rocks raining at the post from the eruption,” said Yulius Ramopolii, the head of Mount Ruang monitoring post. “The vibrations were intense and knocked out power, and volcanic earthquakes shook the glass windows and everything around us.”

He said the eruption blocked out the sun and peppered several villages with falling debris. No casualties have been reported, Ramopolii said.

More than 11,000 people had evacuated after the April 17 eruption when authorities warned that a major eruption might collapse part of the volcano into the sea and cause a tsunami that could endanger nearby villages.

Less than 3,000 remained at temporary shelters after the government lowered its alert level to the second highest from four levels and reopened the airport after four days.

Indonesia's geological agency on Tuesday warned people on Tagulandang Island, especially those who live near the coast, of the potential of hot volcanic clouds and a tsunami due to eruptions of material entering the sea or collapse of volcanic dome into the sea.

Ruang is among about 130 active volcanoes in Indonesia. The archipelagic nation is prone to volcanic eruptions and earthquakes because of its location on the Pacific “Ring of Fire” — a series of fault lines stretching from the western coasts of the Americas through Japan and Southeast Asia.