Prime Minister Shinzo Abe on Monday pledged to keep alive his plans to amend the country's pacifist constitution, despite failing to secure a "supermajority" in upper house elections.

His ruling coalition retained its majority in the upper house in Sunday's vote for around the half the seats in the chamber, but fell short of securing a two-thirds majority in favour of revising the constitution.

"The hurdle of two thirds in both the lower and upper houses is extremely high.

"I want to produce a reform proposal that can be agreed upon by two thirds beyond the borders of the ruling and opposition camps," Abe told a news conference at his party headquarters.