The Pumas are a team of two halves so far at the Rugby World Cup

Argentina has been a team of two halves so far at the Rugby World Cup, and coach Mario Ledesma does not like it.

Poor one half, good the next.

The Pumas led Tonga 28-0 inside 30 minutes on Saturday but failed to score again in a 28-12 win which gave Argentina a bonus point and kept in with a chance of qualifying for the knockout stage.

Against France last weekend, Argentina trailed 20-3 at half-time and stormed back to lead 21-20 before losing 23-21.

"If I knew the reason, this would not happen," Ledesma said through a translator. "I would like to talk to the players in detail about this."

He will need to do that quickly, because Pool C leader England is up next weekend. That's a huge game, and will likely decide if Argentina stays or goes home after the group stage.

"Today's intensity is not enough to play against England," Ledesma said. "With so many missed tackles and losing the ball so much, it will be difficult for us to play against England."

A rout appeared to be on the cards after hooker Julián Montoya's hat trick of tries and one from winger Santiago Carreras — all converted by fly-half Benjamin Urdapilleta and all before Tonga has scored a point.

Instead, Tonga rallied with two fine tries from fullback Telusa Veainu, who looked sharp on his return from a nagging foot injury.

This was a comfortable win, and finally snapped a 10-game losing streak, but the way Tonga got back into the game and pushed hard for a third try concerned Ledesma.

"We need to maintain our focus and concentration," he said. "We lost our focus a bit."

His team changes included dropping mainstay flyhalf Nicolas Sanchez and handing Urdapilleta his first start since June 2013.

Montoya had scored a try as a replacement against France, and now has four after replacing former captain Agustin Creevy in the starting XV.

These might prove the easiest tries he'll get, such was Tonga's poor defending early on. Carreras was also a new pick on the left wing, and the 21-year-old got his first international try.

It all seemed so easy at this point, with not even 30 minutes played.

"Just errors, poor skill, bit of miscommunication. Typical Tier Two errors," said Tonga coach Toutai Kefu, who won the 1999 World Cup as a player for Australia. "You can't let Tier One teams get ahead like that. But I'm proud of how the boys fronted up physically."

But Tonga woke up with a well-taken try from Veainu — a neat side-step into the left corner from the Leicester Tigers favorite.

"He offers us an X-factor out wide," Kefu said. "It's great to have him back. Having a player of his caliber and class is fantastic for us."

Flanker Tomas Lavanini prevented a second Tonga try on the half-time buzzer, barging winger David Halaifonua out of bounds in the left corner. The try was ruled out, although there were questions over the tackle.

"I thought it was a shoulder charge. If we would have got that (try) we would have come out in the second half with a lot more belief and confidence," Kefu said. "There were a couple of 50-50 (decisions) today we didn't get."

After the break, the pace of the contest dropped at a humid 22,000-capacity Hanazono Rugby Stadium.

Argentine scrumhalf Tomas Cubelli had a try ruled out in the 52nd minute for a knock-on, following a referral to the television match official.