Outgoing Prime Minister Shinzo Abe and his Australian counterpart Scott Morrison on Tuesday affirmed the importance of security cooperation between their countries and vowed to strengthen ties under Abe's successor, Japan's Foreign Ministry said.

In a 30-minute phone call, Abe explained to Morrison his decision to resign so he could receive treatment for chronic bowel disease, and thanked the Australian premier for his friendship while in office, according to the ministry.

Abe called Australia an "indispensable security partner" in the Indo-Pacific, and the two agreed that their countries should continue to play a leading role in the region, the ministry said.

Japan and Australia have been stepping up cooperation between their armed forces to counter China's growing assertiveness in disputed waters in the South China Sea.

The two countries, along with mutual ally the United States, in July, carried out joint training exercises near the waters as a show of might.

Japan and Australia have also criticized China for introducing a controversial national security law in Hong Kong that critics say is destroying freedoms and human rights in the semiautonomous territory.