They have pictures of what appear to be very faint galaxies that shone more than 13 billion years ago, a mere 500 million years after the Big Bang.
The remarkable claim dramatically exceeds the current, broadly accepted record for the most distant detection.
The Caltech-led team behind the work recognises there will be sceptics but says it believes its data is strong.
It has published details in The Astrophysical Journal; and the group leader, Professor Richard Ellis, has been arguing the case at a conference in London, UK.
"We've had these galaxies for over a year and we have gone back to the telescope and revisited them, to prove their signals are robust," he explained.
"We feel confident now that we have done all that is humanly possibly to show the community that these galaxies are at these great distances," Professor Ellis told BBC News.
Big zoom
The international team of astronomers found its six "candidate" galaxies using one of the 10m Keck telescope twins sited on Mauna Kea, Hawaii.
The researchers employed a technique known as gravitational lensing to achieve the detections.
This makes use of the gravity of relatively nearby objects (in this case galaxy clusters) to magnify the light coming from much more distant objects (the six candidates). Astronomers from Caltech (California Institute of Technology) have helped pioneer this field; and they say they know how to select just the right "zoom lens" to see back into the required period in cosmic history.
The team then further refined its search by only looking for a very narrow wavelength of light where its target galaxies - if they existed - would be expected to shine.
It has taken three years' painstaking work to make and check the observations.
"Using Keck II, we have detected six faint star-forming galaxies whose signal has been boosted about 20 times by the magnifying effect of a foreground cluster," said Caltech co-worker Dan Stark.
"That we should find so many distant galaxies in our small survey area suggests they are very numerous indeed."
It's a crowd
This is perhaps the most significant implication of the study.
Astronomy is now engaged in a major drive to tie down the timings of key events in the early Universe.
Scientists would like to see extensive evidence for the very first populations of stars. These hot, blue giants would have grown out of the cold neutral gas that pervaded the young cosmos. The behemoths would likely have burned brilliant but brief lives, producing the very first heavy elements.
They would also have "fried" the neutral gas around them to produce the diffuse intergalactic plasma we detect between nearby stars today.
But this theory demands the earliest star-forming phase in the Universe was a busy one - and the significance of the latest study is that it suggests the numbers of stars required did indeed exist.
"The area of sky we surveyed was so small that for us to find anything at all suggests to us these objects must be very numerous," Professor Ellis told the BBC News website.
"Obviously it's a bit of a stretch to estimate a population from just six objects - but if you went out into a London street, looked at one piece of pavement and found six people standing there, I think you could reasonably conclude London was a crowded place."
Redshift hunters
It has been known for a while that the Caltech-led group had some very interesting pictures. In the past 12 months, knowledge of their existence has been shared at scientific conferences and hinted at by popular publications such as Time Magazine and the BBC News website.
But Ellis and colleagues have deliberately held back from formal publication of their work. Theirs is a field which has burned the reputations of others who have rushed forward with announcements that could not be confirmed by subsequent, independent observation.
Even so, to get an idea of how big a leap in detection is now being claimed can be illustrated by the "ruler" astronomers use to describe far-off sightings.
They will often be heard referring to "redshift". It is a measure of the degree to which light has been "stretched" by the expansion of the Universe. The greater the redshift, the more distant the object and the earlier it is being seen in cosmic history.
The current, widely accepted distance record-holder is the IOK-1 galaxy detection announced last year which had a redshift of 6.96. Its light was being seen when the Universe was little more than 700 million years old (Current estimates have the Universe coming into existence about 13.66 billion years ago as a "hot soup" of elementary particles).
Getting to this mark was a process of steady, incremental steps through redshifts in the lower-sixes and fives. The Caltech-led group has now suddenly jumped into the redshift region of eight to 10.
Dr Andy Bunker is a high-redshift hunter with Exeter University, UK. He has worked with the Ellis group in the past but was not involved in this study.
He commented: "Richard is a careful worker and he knows the burden of proof is very high.
"His group is aware of the history of the field and that's why they are being a little bit cagey; but I think this is a significant paper and unlike many that have gone before, I believe this will stand the test of time and at least some of the six candidates will be confirmed by others in due course."
The Caltech-led group hopes soon to get some confirmation of its own by looking at a different wavelength of light using the Spitzer Space Telescope and through the use of a new spectrograph instrument which is being installed at the Keck.
A refurbished Hubble Space Telescope is expected to be able to reach up to redshift 10; and its successor, the James Webb Space Telescope, due for launch early in the next decade, should be capable of redshift 15 observations.
Richard Ellis, the Steele Professor of Astronomy at Caltech, Pasadena, US, delivered a talk at the From IRAS to Herschel and Planck Conference. The meeting was organised to celebrate the 65th birthday of Royal Astronomical Society President Professor Michael Rowan-Robinson.
