Data gathered at the Litli Hrutur peak -- an uninhabited area around 30 kilometres (19 miles) from Reykjavik -- showed that the lava flow was around five cubic metres per second, down from 20 cubic metres per second at the start of the eruption.
The lava flow has declined by almost half in the past week alone, the meteorological office said in a statement.
Data also "indicates signs of deflation" at a depth which likely coincides with the source of the eruption, the office said.
There were therefore "strong indications that the eruption will end soon, but it is difficult to say at this stage whether it is a question of days or weeks."
Thousands of visitors have been flocking to the site to take in the hypnotic spectacle of red-hot lava spurting out of the ground.
The Reykjanes peninsula had been dormant for eight centuries but has experienced a resurgence of volcanic activity in recent years.
There have been two other recent eruptions -- one in the Geldingadalir valley in March 2021, which lasted six months, and one in the Meradalir valley in August 2022, which lasted three weeks.
All of them belong to the Fagradalsfjall volcanic system.
Unlike explosive eruptions that spew out thousands of tonnes of ash -- such as the famous Eyjafjallajokull eruption that paralysed air traffic in Europe in 2010 -- the three recent ones have been so-called "effusive" eruptions and have had little impact, apart from lava flows and locally toxic gas spikes.
Iceland has 33 volcanic systems currently considered active, the highest number in Europe. It has an eruption every five years on average.
The North Atlantic island straddles the Mid-Atlantic Ridge, a crack in the ocean floor separating the Eurasian and North American tectonic plates.
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