"If Thailand is leading the global crusade, I don't put a lot of stock in its success," quipped the US official, who asked not to be named.

Abbott's office in Bangkok is forwarding all media requests to its headquarters in Chicago.

Communications staff there, however, are not allowed to answer questions. They have been instructed to telephone journalists who request information from their Bangkok office and read two sentences over the phone.

"You have to quote me exactly," Dirk Van Eeden, a communications director for Abbott International, said before reading the statement: "Thailand has chosen to break patents on numerous medicines, ignoring the patent system. As such, we've elected not to introduce new medicines there."

He said he could not explain how Thailand had ignored the patent system or provide information because of the "particularly sensitive" situation Abbott was in.

He did not explain why his colleagues in Bangkok were not permitted to read the sentences.

An Abbott communications officer in Bangkok said she could not answer questions "in case she made a mistake".

Meanwhile, the Ministry of Public Health has released a 96-page document explaining its position on the issue, including the legality of its move under global trade rules and the numerous precedents it had followed.

The document includes letters of support from the Clinton Foundation, UNAids, 22 US Senators and the World Health Organisation, among others.

In January the ministry used a mechanism provided by global trade rules to override a patent on an HIV/Aids drug produced by Abbott so that it could import or produce a generic version to those who could not afford the patented one.

This will save 8,000 Thai lives, the ministry said.

Spending on HIV/Aids medications in Thailand has surged tenfold over the past six years. Thailand's effort to provide universal access to the drugs is not sustainable and the country needs greater access to generic versions, studies by Thai and international agencies show.

Thailand began issuing so-called "compulsory licences" on life-saving medications in November last year, following a seven-year lobbying effort by local and international non-government organisations.

The mechanism is enshrined in global trade rules and has frequently been used by

other countries. Abbott faced a similar dispute with Brazil over a threat to use a compulsory licence for the same life-saving drug there.

Meanwhile, the company is now facing criticism from a coalition of Christian shareholders who say its decision to retaliate against Thailand over the patent dispute could damage the value of its stock as well as the image of its brand.

Abbott's decision not to introduce new medicines in Thailand has been denounced by medical associations as a violation of medical ethics.