The move to increase it from three to four, with five being the maximum, means there is a "high threat" of an attack.

The Swedish PM Ulf Kristersson told a news conference: "We know that planned terror attacks have been averted." But he did not provide further details.

It comes after copies of the Koran were recently burned in the Scandinavian nation by a handful of anti-Islam activists.

This sparked angry demonstrations in countries, including Iraq, Iran, Lebanon and Yemen, by Muslims who consider such acts to be blasphemous to their holy book.

Sweden and neighbouring Denmark, which are among the most liberal countries in the world, have tightened border controls in fear of revenge attacks.

The US has warned its citizens to be extra cautious when travelling to Sweden due to possible terrorist attacks.

Like many Western countries, Sweden doesn't have blasphemy laws that ban the burning of religious texts.

The nation has switched from being a "legitimate target for terrorist attacks" to being considered a "prioritised target", SAPO security service chief, Charlotte von Essen, said.

Ms von Essen added: "The increase in the terror threat level is being done to raise consciousness that the threat we see will remain for a while and society needs to build up its resistance to meet that."

The attack threat posed by "violent Islamist actors" had risen in the past year but the increased level was not down to knowledge of a specific plan, she said.

The Swedish military is also raising the terrorism threat levels for their operations.

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In June, Salwan Momika, a Christian man who immigrated from Iraq to Sweden, burned a Koran in front of Stockholm's largest mosque on the first day of the Muslim holiday of Eid.

Last month, he staged another protest in Stockholm, stomping on and kicking a Koran outside the Iraqi embassy, although he refrained from burning it.

Also, the "Danish Patriots", a far-right group, set fire to the holy book outside the Iraqi embassy in Copenhagen in July.

In 2017, Sweden suffered its worst terror attack in recent history when five people were killed by Uzbek asylum seeker Rakhmat Akilov, who pledged allegiance to the Islamic State.

He used a truck to knock down pedestrians on a busy street in Stockholm.