The reigning Olympic champions and world number one side triumphed 25-27, 25-21, 27-25, 25-22 at Nagano's White Ring to make it five victories from five.

They are the only side to remain unbeaten as they bid for a third World Cup, and a first since 2007.

Yoandy Leal Hidalgo and Alan Souza both scored 16 points for the Rio 2016 winners as Porya Yali's huge haul of 25 for Iran was in vain.

"Five victories is very important for us, but we are still in the middle of a long tournament so we can't say anything yet about what will happen at the end or whether we will become champions," said Brazil's head coach Renan Dal Zotto.

"All I can say now is that we must keep our focus and intensity. 

"If we allow ourselves to relax too much it will be a disaster for us."

All 12 teams at the World Cup are playing each other in a round-robin group, with the table toppers declared the winners.

World Cup holders United States and reigning World Championship winners Poland remain as Brazil's closest challengers with both on a record of four wins and one defeat.

The Americans eased past Tunisia at Marine Messe Fukuoka today, 25-10, 25-18, 25-17.

Poland beat world number two side Italy 25-18, 25-18, 25-22 at the same venue.

Hosts Japan and Russia also both won today to sit on records of played three, lost two.

The home side saw off Argentina, who shocked the US in their opening match, 25-19, 25-20, 26-28, 25-22 in Nagano.

Russia beat Egypt 25-19, 21-25, 25-19, 25-21 in Fukuoka.

Canada proved too strong in Nagano for Australia, coming through 18-25, 28-26, 25-20, 25-22.

The Canadians are one of five teams on a won two, lost three record, alongside Egypt, Argentina, Iran and Italy.

Australia have one victory with Tunisia yet to get off the mark.

There will now be two rest days as the World Cup moves to Hiroshima, where the remaining matches will be played.

Two courts at the Hiroshima Prefectural Sports Center will be used.