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Countries including New Zealand, Australia and the United States are to provide support to Tonga after a tsunami hit the South Pacific island nation.

The waves were caused by Saturday's eruption of the underwater Hunga Tonga-Hunga Ha'apai volcano, the second in two days.

The eruption caused a huge plume of ash, steam and gas to rise from the ocean.

The large waves prompted also tsunami alerts to be issued to countries as far away as Chile and Japan, although scientists say that the risk of further tsunamis is now over.

New Zealand Prime Minister Jacinda Ardern said that there had been no reports of anyone being injured but that Tonga's capital Nuku'alofa had suffered "significant" damage.

She added that a full assessment was not yet possible as the eruption and tsunami had caused communication lines to go down in the country.

"Nuku'alofa is covered in a thick film of volcanic dust but otherwise conditions are calm and stable," she said.

New Zealand's Armed Forces tweeted their support, "We're working hard to see how we can assist our Pacific neighbours after the volcanic eruption near Tonga."

Videos shared on social media showed waves more than a metre high flooding houses and streets in Tonga's capital, Nuku'alofa.

The city lies around 40 miles north of the volcano, on the country's main island of Tongatapu.

People on the island were asked to move to higher ground in order to protect themselves from the tsunami.

The giant eruption of the Hunga Tonga-Hunga Haʻapai volcano was heard across the South Pacific, reportedly as far as 6000 miles away in the American state of Alaska.

Prof Shane Cronin, a volcanologist at the University of Auckland in New Zealand, said the eruption was one of the biggest in Tonga in the past 30 years.

"This is a pretty big event - it's one of the more significant eruptions of the last decade at least," he told the ´óÏó´«Ã½.

"The most remarkable thing about it is how rapidly and violently it's spread," he added.

A tsunami is a large sea wave, sometimes taller than a house, which can cause huge amounts of damage.

They can be caused by underwater earthquakes or volcanic eruptions, and can happen unexpectedly and be very unpredictable.

Tsunami is a Japanese word which means 'harbour wave'.