Hundreds of parents, mostly men, from North America, Europe and elsewhere have been left without any recourse after their estranged partners took their half-Japanese children back to the country.

Unlike Western nations, Japan does not recognise joint custody and courts almost always order that children of divorcees live with their mothers.

US lawmakers have long demanded action from Japan on the issue, one of the few open disputes between the close allies. In February, Prime Minister Shinzo Abe promised action after White House talks with US President Barack Obama.

The upper house of parliament on Wednesday voted unanimously for Japan to join the treaty, following a similar move by the more powerful lower house last month.

But Japan must still clear various governmental and legislative hurdles before the Hague Convention can take full effect.