Tonga's defence and foreign ministers resigned their portfolios Thursday, the climax of a months-long constitutional crisis that has seen the Pacific nation's powerful King bring his elected government to heel.

Prime Minister Siaosi Sovaleni announced in parliament that he would relinquish his armed forces portfolio and top ally Fekitamoeloa 'Utoikamanu would resign her foreign affairs and tourism portfolios -- bowing to royal demands.

Tonga has been a constitutional democracy since the late 19th century. The royals' role in politics has receded since 2010, when democratic reforms saw the monarchy relinquish some of their powers.

But that trend was challenged this year, when King Tupou VI abruptly withdrew "confidence and consent" for appointments to three key ministries.

No reason has been publicly given, but speculation has swirled about a power play between hereditary royals and the political class.

Malakai Koloamatangi, a political scientist at the University of Fiji, said the crisis should be seen as part of long-running unreconciled tensions "between the position of the monarch and parliament-executive".

"It's unclear what the motivations are on the part of His Majesty" Koloamatangi said, "it could be that he thinks that the defence... portfolio ought to be in the hands of a noble in accordance with past precedent."

Initially, the prime minister had refused to budge.

Legal advice from Tonga's attorney-general claimed the King's move was unconstitutional.

The government cited a constitutional clause that ministers are removed by the King "on the recommendation of the prime minister" or through a parliamentary vote of no confidence.

The prime minister's U-turn came after fierce debates in parliament and after he denied allegations of insulting the King.

The tourism portfolio had been reassigned, but no new minister for either the armed forces or foreign affairs had yet been appointed, the prime minister added.

Tonga is home to around 100,000 people spread over 170 islands and claims to be the only remaining indigenous monarchy in the Pacific.