The eruption began ad 0746 GMT and continued for about a half hour, according to Marta Calvache, an official from Colombia's Institute of Geology and Mines.

"It was a small eruption that spewed ash around the volcano," Calvache said. "There was also a jolt that was felt in the region, but it was not strong."

However, she said it was not clear how Galeras will behave in the future, and urged local residents to be cautious.

A layer of ash covered the town of Pasto, but Mayor Raul Delgado said the situation was under control.

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