A United Nations drug enforcement body has voiced concern about the possible smuggling of a powerful synthetic tranquiliser into Thailand and the Southeast Asian region after it was spread across the US and Europe using postal services.

Viroj Sumyai, president of the International Narcotics Control Board (INCB), said surveillance is being stepped up on the trafficking into the kingdom of carfentanil, a tranquilising agent for elephants and other large mammals which is now being used to cut narcotics.

The drug dealers mix the synthetic drug with heroin, which further increases the danger and impact on users. US health authorities last year reported 10,000 fatal cases of carfentanil-laced heroin overdoses and declared it a health crisis, he noted.

According to Mr Viroj, the synthetic drug is traded online and shipped overseas through the postal system.

The INCB recently signed an agreement with the Universal Postal Union (UPU) in an attempt to curb the trafficking of carfentanil via letters and e-commerce packets, he said.

The INCB is an independent body monitoring the implementation of the United Nations' international drug control conventions.

The agreement is expected to promote technical assistance and increase cooperation against trafficking and improve the detection and seizure of these substances, he added.

However, he said carfentanil is traded online using so-called dark-web markets and cryptocurrencies to maintain anonymity in the illicit drug trade. This makes it difficult for authorities to trace the transactions.

"In Thailand we've increased surveillance to determine if the drug has made its way into the country. The substance is extremely dangerous and can be absorbed through the skin, so it must be handled with care," Mr Viroj said.