Japan on Tuesday said it stood ready to join the United States and other Group of Seven industrialized nations in slapping sanctions on Russia, should President Vladimir Putin order an invasion of Ukraine.

The confirmation by the world's No. 3 economy comes as the crisis in Europe deepens, with the Russian leader on Monday ordering troops into two breakaway regions in eastern Ukraine that Russia now recognizes as independent states.

That action was "unacceptable and a violation of international law," Prime Minister Fumio Kishida, who called Putin on Thursday to urge restraint, told reporters. Japan was ready for a strong response that could include sanctions, he added.