The volcano on uninhabited Anatahan Island in the Northern Mariana Islands (CNMI) became active again Tuesday.

"Although the volcano is not currently dangerous to most aircraft within CNMI airspace, conditions may change rapidly, and aircraft should pass upwind of Anatahan or farther than 30 kilometres downwind," the Emergency Management Office (EMO) said in a statement.

EMO director Rudolfo Pua said the size of the plume was getting higher.

"It may become a major eruption, we don't know yet," Pua said.

"We just want airlines to be on alert again."

It is the fourth eruption of Anatahan since it suddenly burst into life in May 2003, sending smoke and ash 30,000 feet into the air.

EMO seismic technician Juan Camacho said three airlines -- Continental Micronesia, Air New Zealand and Japan Airlines -- had confirmed the latest eruption was reaching 15,000 feet.

Under an emergency declaration, Anatahan has been declared off limits to all but scientific expeditions until the end of January.

Anatahan is a 33 square kilometre (13 square miles) island, 128 kilometres (80 miles) north of here and just over 322 kilometres (200 miles) north of Guam. It is around 2,250 kilometres (1,400 miles) south of Tokyo.

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