TOKYO (AFP) - Japanese Prime Minister Shinzo Abe won comfortable re-election on Sunday in a snap poll he had billed as a referendum on his economic policies, but a record low voter turnout threatened to cloud any mandate.

Despite only around half of voters casting a ballot, the conservative Abe claimed popular endorsement for his mix of nationalism and "Abenomics" - a signature plan to fix the country's flaccid economy that enjoyed early success but faded into a recession.

"The ruling coalition has been given a majority," Abe told a television interviewer. "We humbly want to meet the public's expectations.

"I think we received people's mandate for the Abe government's performance in the past two years. But we must not be complacent and must carefully explain to the public when implementing policies," Abe told TBS.

Media exit polls showed his ruling Liberal Democratic Party (LDP) and its junior partner Komeito had swept the ballot, with an unassailable two-thirds majority in the Lower House of Parliament, giving them the power to override the upper chamber.

TV Asahi said the pairing had won 333 of the 475 seats, while TBS put the figure at 328.

Washington was among the first to welcome Abe's victory, hailing his "strong leadership on a wide range of regional and global issues" from Ebola to the fight against the Islamic State group.

"The US-Japan alliance is the cornerstone of peace and prosperity in the Asia-Pacific," White House spokesman Josh Earnest said in a statement.

Abe, 60, was only halfway through his four-year term when he called the vote last month.

The first two of his "three arrows" of Abenomics - monetary easing and fiscal stimulus - have largely hit their targets; the once-painfully high yen has plunged, giving exporters a boost, and stocks have rocketed.

Prices have also begun rising after years of treading water - proof, says Abe, that this is the beginning of a virtuous circle of economic growth, with higher wages soon to follow.

However a sales tax rise in April snuffed out consumer spending, sending Japan into the two negative quarters of growth that make a recession.

'Economy is my priority'

Economists say more important than the sugar rush offered by easy money and government spending is structural reform of Japan's highly-regulated and protected economy - the third arrow of Abenomics.

Abe has been criticised for not being bold enough in taking on the vested interests that are the real key to reversing nearly two decades of economic underperformance.

His fresh four-year mandate may stiffen his resolve for these reforms and see off opposition from within the fractious LDP, a party given to bouts of regicide.

"This victory will enhance Abe's political capital and allow him to tackle tough issues more comfortably," said Yoshinobu Yamamoto, professor of politics at the University of Niigata Prefecture.

Some worry that freed from the constraints of elections, Abe may nurse his pet nationalist projects, such as revamping the pacifist Constitution and urging a more sympathetic view of Japan's warring past.

"Economy is my first priority," he told NHK. "Then through the strategic diplomacy that takes an overview of the globe, I'll enhance Japan's position." But with only around 52 per cent of voters casting ballots - down seven percentage points on 2012 - there may be questions over whether Sunday's result really is an endorsement, or just the default reaction of an electorate numbed by a lack of viable alternatives.

Voters polled in the run-up to the ballot were uninspired by the choices on offer.

Only two-thirds of respondents told Kyodo News earlier this week that they were interested in the election.

Many Japanese were bitterly disappointed by three years under the Democratic Party of Japan from 2009, which saw three emasculated prime ministers and a series of policy flops.

The Communist Party, which consistently opposed nuclear restarts and tax raises, and is one of the few parliamentary groupings with a discernable ideology, was a notable winner on the evening, more than doubling their tally of seats to at least 20.

An extraordinary Diet session is expected to be convened on Dec 24, when Abe must be formally named prime minister by the Lower House.