Tightened US visa requirements are disrupting international scientific collaborations and driving academic conferences and many foreign graduate and doctoral students overseas, some to the UK.
Entry restrictions brought in after the September 11 2001 terrorist attacks have led to visa applications made by scholars from many nations being either refused or severely delayed.
Many academics and students from the Middle East are thought to have stopped applying, while growing numbers from China and Russia are finding it difficult to accept university places, work on international projects or attend conferences in the US.
Peter Schindler, executive secretary of the International Council for Science's standing committee on freedom in the conduct of science, said this had led to a surge in applications for academic positions in Europe.
"I can understand the Americans were frightened but, in our opinion, they are overreacting and putting everyone in the same pot," Dr Schindler said.
As a result, the ICS has called on its member bodies, which include 75 national academies and scientific unions, to hold their conferences outside the US if possible.
Monique Orine, executive assistant at the International Astronomical Union, confirmed that the IAU had declined an invitation to hold a future general assembly in Hawaii because of the problem.
Among those affected is Sudanese-born Mohamed Hassan, chief executive of the Third World Academy of Sciences, whose application for a visa has been delayed for eight months.
Another case involves a senior British scientist who has worked at a US university for years yet cannot attend meetings abroad as she would then need to apply for a visa - with a likely wait of several months - to reenter the US.
The woman, who said she did not want to be named, said: "It is extremely serious in terms of the US scientific enterprise since so many postdocs and research assistants in US labs are from overseas - not to mention faculty."
Among the US-based collaborations to have already suffered is the particle physics project D0 at the Fermi Lab - many team members, especially Chinese and Russians, cannot enter the country.
D0 spokesman John Womersley said those already involved in US collaborations would probably grit their teeth. But he feared that many would soon look elsewhere to pursue their work. "The restrictions are getting stricter, and no one outside the scientific community seems to understand the cost of this," he said.