Sweden's top diplomat Tobias Billstrom announced his resignation in a surprise move on Wednesday and said he would leave politics altogether.

The 50-year-old said in a post to X that with a mix of "sadness and pride" he had informed Swedish Prime Minister Ulf Kristersson of his resignation.

Billstrom, who has served as foreign minister in Kristersson's cabinet since the coalition government assumed office in October 2022, did not provide any reason for his decision.

"For my part, I will now leave politics entirely. This means that I am also leaving my parliamentary seat," Billstrom said, adding that he would formally leave his position when Sweden's parliament re-opens next week.

"What I will do next is still open. But I am only 50 years old and I look forward to contributing and working hard in other contexts," Billstrom said.

During his tenure as the country's top diplomat, Sweden completed its accession to NATO after a dragged-out process to convince Türkiye and Hungary, whose ratifications were needed for Sweden to become a member of the U.S.-led military alliance.

Sweden abandoned two centuries of military non-alignment and applied for membership in NATO in the wake of Russia's invasion of Ukraine.

Billstrom noted that "we are in the most serious security situation since the Second World War," while citing the war in Ukraine and the situation in the Middle East.

"There is no shortage of tasks for my successor, whom I wish the best of luck," Billstrom said.

In a statement on Facebook, Kristersson wished Billstrom well in his "future endeavors," but did not immediately name any potential successor.

Billstrom's resignation coincided with the resignation of Ukrainian Foreign Minister Dmitro Kuleba, amid a major cabinet reshuffle.