"If an airline or other transport provider refuses to take a shipment, or is unable to take a shipment, then this increases the prospect of someone missing a cancer treatment," Michael Wangler, head of Safety of Transport of Radioactive Materials at the International Atomic Energy Agency (IAEA), said.

"There is a risk that if more airlines do deny, particularly where few airlines serve key regions, then this does raise a serious issue," Wangler said in a statement. "It potentially means that medical clinics and hospitals in specific areas are at risk of being denied essential medical supplies."

Most countries import isotopes commonly used in medicine, such as those used to treat cancer, diagnose heart attacks or sterilize medical equipment, the IAEA said.

Hospitals and clinics depend on these international shipments to arrive on time, especially if the isotope has a short half-life and must be sent by air, the agency said.

But since the September 11, 2001 attacks, worries about the potential threat of a dirty bomb attack - in which common explosives would be laced with medical or other radioactive material - have caused transporters to crack down on radioactive shipments.

Shipments of iodine, which has a very short half-life and is needed to treat thyroid cancer, have been refused by airlines, rendering it useless and endangering patients, the IAEA said.

Refusing to carry such material has become "the path of least resistance," a Vienna diplomat who follows IAEA issues told Reuters.

Shipments of medical and industrial radioactive material are regulated by countries and the airline industry in accordance with the IAEA regulations.

"Radioactive material is very safely transported, based on standards developed by the IAEA which have been operating for 43 years," Wangler said. "What the current regulatory system lacks are enhanced efforts or separate treatment to facilitate the rapid distribution of medical isotopes when warranted."

The IAEA said the July meeting on this issue was part of an international fact-finding campaign to understand the full extent of the problem and propose solutions.

Participants represented regulatory authorities, producers of radioactive sources, airlines, shippers and other transport operators, and international organizations.