Twenty-two people were treated for heat-related illnesses on the first day of Houston's World Cup fan festival as supporters struggled to deal with the rising temperatures across America.

Of those who received medical attention, the condition of four supporters was so severe that they were transported to the hospital for further treatment.

According to Harris County Judge Lina Hidalgo, at least two cases involved heatstroke, and she called for more help to be given to those attending FIFA's fan festival in the city.

'We need more shade,' she said. 'There are two areas that really have air conditioning. It’s the Houston Hall and then there is the NASA bubble.'

She also raised concerns about the lack of hydration stations, and misting stations not keeping people sufficiently cool in the boiling sun.

Hot and humid conditions are due to continue through the weekend in Texas, with temperatures in the high 90s (35c +), and heat indexes in the 100s.

In response to concerns from the county judge, Juan Garcia - chair of the Houston Sports Authority - hit back and urged visitors to 'come prepared' and take breaks from the heat.

Advice has been dished out to visitors not used to the searing heat of an American summer - with games taking place in the south particularly affected.

'I will tell you, one of the big concerns or challenges we have is just our weather over the next 39 days,' Brian Mason, the director of the Houston Officer of Emergency Management, said. 

'Especially, we're going to have a lot of out of towners visitors who aren't used to our heat and humidity.

'So our real kind of ask, or words of wisdom for people is please make sure you’re paying attention to your body. Please make sure you hydrate, and hydrate, and hydrate some more.'

Houston's World Cup fixtures kick off on Sunday when one of the tournament favorites Germany take on Curacao.

A few days later, Cristiano Ronaldo is in town as his Portugal face Congo DR, before the Netherlands vs Sweden on June 20.

On June 23, Portugal are back to play Uzbekistan before the group stage action concludes with Cape Verde vs Saudi Arabia.

There will then be two as-yet-unconfirmed knockout matches, in the Round of 32 and then Round of 16, with the final game taking place on July 4.