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Defence coach Shaun Edwards has warned Wales' new-look squad that the tour to Japan will be a "huge test".

Wales, with the bulk of their frontline players on British and Irish Lions duty, face Japan in a two-Test series in Osaka and Tokyo in June.

Edwards, who lost out to England's Andy Farrell to be Lions defence coach, is under no illusions of the task facing Wales in the Far East.

"It is definitely a huge test for us," said Edwards.

Wales travel to Japan as the reigning 2013 Six Nations champions, but with 15 Welsh players missing due to Lions duty in Australia.

Warren Gatland, the Wales coach, and Rob Howley, his 2013 Six Nations deputy and Lions attack coach, are also with the Lions heading down under.

Robin McBryde, Wales' caretaker coach for the Japan tour, has also left experienced players Ryan Jones, Matthew Rees, Paul James and James Hook out of his squad, while Rhys Priestland, Scott Williams, Ken Owens, Aaron Shingler, Ashley Beck and Eli Walker are ruled out by injury.

"Japan are an improving nation [and] there are lots of excuses with the heat and the humidity and the fact that we have approaching 30 players missing" said Edwards.

"But we seem to have a resolute bunch of lads.

"They have got up to speed in training the last couple of weeks.

"As a coach you work harder because you have more to teach with more inexperienced and younger players.

"The attitude has been fantastic and I am impressed with their improvement."

Japan prepared for the visit of Wales with a 27-17 defeat by Tonga in Yokohama in their first 2013 Pacific Nations Cup match last Saturday.

The Cherry Blossoms travel to Fiji on Saturday before the games against Wales in Osaka (8 June) and Tokyo (15 June).

Japan had previously steamrollered their way through the Asian Five Nations, scoring 316 points and conceding just eight.

But Edwards says the conditions, temperature and humidity - Wales are playing in the stifling mid-afternoon heat in Japan - are also major concerns for his young and inexperienced squad.

"We have done a couple of sessions in the [Wales training] barn and turned the heating up - it was like Benidorm," said Edwards

"But seriously, we did turn the heating up and did some conditioning sets and the guys have had the heating really hot in the gym.

"They have been really pushing themselves - with power and endurance - in the heat, trying to get used to it, but it is difficult."

And the former Wigan and Great Britain rugby league star also recalled his own experiences of playing in extreme heat.

He said: "I played in Papa New Guinea in 1992 and it was one of the hardest things I have ever done in rugby.

"It was really, really tough and I just hope Japan is not as hot as Papa New Guinea. It was... 38 and 40 degrees."