The measurements, by the first comprehensive study ofocean expansion, will add to concern that global warming is taking place, and add urgency to calls from low-lying island and coastal nations for cuts in the pollution that causes it.
The calls will come to a head next month when 100 nations meet in Berlin to decide whether to tighten up the international treaty on climate change agreed at the 1992 Earth Summit.
Scientists have long predicted that sea levels will rise as the world heats up. By their best estimates, the waters have been rising by about one-and-a-half millimetres a year over past decades. But preliminary readings by the Topex/Poseidon satellite suggest that the seas have been swelling by three millimetres over the past two years.
The satellite has taken half a million readings every day from the world's seas and oceans, producing much more precise results.
Dr Steven Nerem, of Nasa, says the measurements have "the precision we need to measure detailed sea level and climate changes". But, he stresses, it is too early to come to any final conclusions.
He added that if the same rises were found over the next few years, it would show that global warming had started. This would have serious consequences. Even under the old estimates, five island nations - the Maldives, Tuvalu, Tokelau, the Marshall Islands and Kiribati - are expected to disappear under water over the next century.Reuse content