Norwegian Prime Minister Erna Solberg late Thursday announced that her centre-right government was to expand to include the Christian Democrats.

"It's a historic day," she told a televised news conference after a new government platform was approved by four parties that will form the coalition.

The addition of the Christian Democrats secures a parliamentary majority for the new coalition.

The previous coalition comprised Solberg's conservatives, the right-wing populist Progress Party and the Liberals.

Solberg, who has been prime minister since 2013 and was re-elected in 2017, said the platform secured after 14 days of talks would address climate action, creating a viable welfare society and security.

"We will complete the police reform, strengthen the ability of the police to tackle cyber crime and continue to strengthen Norway's defence capabilities," Solberg said.

The Christian Democrats have often been at odds with the Progress Party, including over immigration and asylum policies.

Progress Party leader Siv Jensen, who is finance minister, said her party had been assured that a "restrictive and responsibile immigration policy would remain in place."

The Christian Democrats were split over joining the government, but an extraordinary party conference in November voted 98-to-90 to open talks.

A slim majority, 19-to-17, in the party board endorsed the platform.

Deputy leader Kjell Inge Ropstad said the Christian Democrats had secured "important" commitments to tackle poverty among families with children.

The Christian Democrats were to elect a new leader in April since party leader Knut Arild Hareide stepped down Thursday. He had urged the party to shift allegiance and support a left-leaning coalition led by the opposition Labour Party.