TOKYO, April 10 -- Japan downgraded its assessment of China on Friday for the first time in a decade, marking a new blow to relations between the Asian superpowers.

Japan's ties with Beijing have soured in recent months after Prime Minister Sanae Takaichi hinted in November that Tokyo could intervene militarily in any attack on self-ruled Taiwan.

China views the island as its territory and has not ruled out taking it by force.

The Japanese foreign ministry's bluebook - which details Tokyo's official views on diplomacy and the international climate - had described China as "one of Japan's most important" partners since 2016.

But this year's edition simply calls China "an important neighbour".

The report accused Beijing of "strengtheni...