Japanese Foreign Minister Kamikawa Yoko has expressed deep concerns about China's expanding military activities. She has also stressed that Beijing's actions need to be monitored closely.

The Stockholm International Peace Research Institute released its annual report on Monday. In the document, it estimates that China now has 500 nuclear warheads.

The institute says that is 90 more than the figure it reported last year. It adds that the number is expected to grow.

Kamikawa told reporters on Tuesday that China has continued to keep its defense budget at a high level. She said the country is rapidly expanding its nuclear, missile and other military capabilities, but she noted that Beijing's actions lack transparency.

The foreign minister said this is an extremely serious matter for Japan and the international community. She added that China's actions need to be carefully watched.

China's nuclear buildup was among the topics discussed during the working-level talks between the Japanese and US governments that were held in the United States last week.

The two sides agreed that China needs to acknowledge that it is rapidly building up its military.