Shinzo Abe's party is forecast to win more than 63 seats, or a majority of the 124 seats

Prime Minister Shinzo Abe's ruling coalition is on course to grab a solid majority in Japan's upper house elections this month and cement his premiership, local media reported on Saturday.

Abe's Liberal Democratic Party (LDP) and its coalition partner Komeito are forecast to win more than 63 seats, or a majority of the 124 seats contested in the July 21 election, Kyodo News said in its opinion poll.

The ruling parties "stand a good chance of reaching 77," said Kyodo, which conducted the poll on Thursday and Friday.

The Asahi Shimbun and other newspapers also said the ruling coalition was expected to win "a comfortable majority" in the election.

The two parties control 70 seats in the other half of the upper house that is not contested. This means that according to the projection they are set to secure their majority in the 245-seat body.

Parliament voted last year to increase the number of seats in the chamber by three from the current 242.

The House of Councillors, as the upper house is formally known, is the less powerful chamber in Japan's bicameral parliament, and half its seats come up for election every three years.

Abe, who has been seeking to revise the pacifist constitution since he took office in late 2012, is aiming to secure a two-thirds "super majority" needed to revise the charter in the election.

"This is an election to decide whether to pick lawmakers and parties who take responsibility for discussions" on constitutional revision, Abe told voters in a campaign speech on Thursday.

Local media predicted pro-revision forces, led by Abe's LDP, are approaching 85 of the seats being contested to have a two-thirds majority in the chamber.

Japan's constitution, imposed by the United States forces after World War II, prohibits the country from waging war.

The provisions are popular in the public at large, but reviled by nationalists like Abe, who see them as outdated and punitive.

Apart from a two-thirds majority in both houses, constitutional revision also requires a majority in a national referendum.

A planned consumption tax hike in October and the nation's fragile pension system are also among key issues for candidates during the run up to the election.

Abe has emphasised the nation's diplomacy, including his "successful" chairmanship of last month's G20 summit in Osaka, which drew US President Donald Trump and other world leaders.