TOKYO (REUTERS) - Japanese Prime Minister Shinzo Abe's ruling bloc suffered two rare losses in by-elections on Sunday (April 21) in an apparent warning from voters not to get complacent ahead of a national election for Parliament's Upper House later this year.

The defeats in a Lower House by-election in Osaka, western Japan, and another on the southern island of Okinawa - host to the bulk of United States military in the country - were the first such losses since Mr Abe returned to office in December 2012, except for one uncontested poll.

"Each individual (ruling) Liberal Democratic Party member must take the results to heart and buckle down," Mr Abe told reporters on Monday morning.

The defeats in the Sunday polls come after Japan's Olympics minister Yoshitaka Sakurada resigned a year before the Tokyo Games for remarks that offended people affected by the massive earthquake and tsunami that triggered nuclear meltdowns in 2011.

A vice-transport minister also quit over a separate gaffe.

"The Cabinet support rate is maintaining a certain level, but if they do not eradicate laxity and conceit, the Upper House election will perforce be a difficult fight," said an editorial in the conservative Yomiuri newspaper.

Support for Mr Abe's Cabinet was at 47 per cent in a survey by public broadcaster NHK released this month, up five points from the previous month.

In Okinawa, Mr Tomohiro Yara, a freelance journalist backed by several opposition parties and running on an anti-US base platform, defeated a former Cabinet minister.

In Osaka, Mr Shimpei Kitagawa, backed by the LDP and its junior partner Komeito, lost to Mr Fumitake Fujita from Nippon Ishin (Japan Innovation Party), a conservative Osaka-based party that sometimes cooperates with the LDP nationally.

Speculation is simmering that Mr Abe will call a snap Lower House election in tandem with the Upper House poll, possibly after announcing the postponement of a sales tax hike to 10 per cent from 8 per cent, scheduled for October.

Top government officials vowed last Friday to go ahead with the tax rise, barring a big economic shock.

Such a "double election" might help take advantage of weakness among the fragmented opposition parties, but could also spark the opposition to cooperate on candidates.

"Abe must be wondering which suffers more from weakness - LDP/Komeito or the opposition," said Sophia University political science professor Koichi Nakano.

"A double election will also potentially galvanise the opposition into action... so it's a double edged sword."