WASHINGTON (AFP) - US officials announced the largest-ever bust of a global synthetic drugs ring on Wednesday, seizing thousands of pounds of illicit drugs and arresting 225 people in five countries.

Authorities seized up to 1,500kg of "dangerous designer synthetic drugs" that were manufactured in Asia, notably China and India, and trafficked to the United States and Australia, where dealers sold them to youths and young adults, the US Drug Enforcement Administration (DEA) said.

The DEA described Project Synergy, which began last December but culminated on Wednesday when most of the arrests were made, as the largest synthetic designer drug bust in law enforcement history."

The drugs are part of a swelling group of illicit compounds that traffickers have begun marketing in the West in recent years, often attempting to skirt laws by barely modifying the chemical makeup of products like incense, bath salts or jewelry cleaner.

The drugs - synthetic cannibinoids that can provide a marijuana-like high, or synthetic cathinones that are stimulant hallucinogens - are marketed under brand names like "Spice," "K2," "Vanilla Sky" or "Bliss" and sold in colourful, youth-oriented packaging, often with comic book characters on the cover.

US officials say abuse of the drugs can lead to vomiting, seizures, hallucinations, high blood pressure, organ damage and loss of consciousness.

Several overdoses, mainly of people between age 12 and 29, have led to emergency room visits and even deaths, the DEA said.

"The bottom line is that these drugs are being marketed to the most vulnerable part of our society, which is teenagers and young adults," DEA chief of operations James Capra told reporters in announcing the bust.

He said millions of dollars in profits from the drug trafficking were being funneled to groups in the Middle East. While Mr Capra would not be drawn on exactly which groups were involved, citing the ongoing investigation, he strongly hinted that terror networks were involved.

"You have this convergence out there, more so today than ever before, of terror groups funding their operations through the sale of narcotics around the globe," Mr Capra said.

Officials highlighted the international nature of the operation, particularly the involvement of agents in Australia, where the synthetic drugs have been marketed.

The seizure "is a terrific result for law enforcement agencies across the globe," said Australia's Acting Ambassador to the United States Graham Fletcher.