Pictures from the scene of the eruption show the epicentre of the new fissure located in between the two points of the earlier eruptions, billowing out smoke and molten rock.
The new river of bright orange magma flowed down the slope to join an expanding field of lava at the base, now covering more than 33 hectares (81 acres), according to the last press briefing by the Icelandic Meteorological Office late Tuesday.
The site had been closed to the public Monday because of the new activity, then reopened early Wednesday.
Icelandic authorities have been forced to issue a public warning as the spectator natural event became a must-see attraction for legal thrill-seekers.
The volcanic eruptions at Mount Fagradalsfjall in Iceland have attracted a growing number of tourists, eager to catch a glimpse of the spectacle.
Thousands of Icelanders have flocked to the site of the eruption some 30 kilometres southwest of the capital.
“It’s a perfect tourist eruption,” volcanology professor at the University of Iceland, Thorvaldur Thordarson, told Reuters.
“With the caveat though, don’t go too close.”
Icelandic Prime Minister Katrín Jakobsdóttir had urged caution in anticipation of mass gatherings.
Ms Jakobsdóttir had previously tweeted: “A volcanic eruption has begun in Fagradalsfjall on the Reykjanes peninsula.
“We are monitoring the situation closely and as of now it is not considered a threat to surrounding towns.
“We ask people to keep away from the immediate area and stay safe.”
The eruption is the first one in the Reykjanes Peninsula for 781 years and happened after geologists detected more than 50,000 recent earthquakes
In 2010, the explosive eruption of Eyjafjallajökull captured the attention of the world.
Volcanic ash from the eruption caused havoc to air travel across western and Northern Europe throughout the spring and early summer.