The Hong Kong Monetary Authority has implemented 24-hour monitoring of the foreign-exchange market following unusual trading activity by overseas banks. The authority stepped up its vigilance amid concern currency speculators could be looking for an opportunity to attack the linked exchange rate. In the past 12 months, the authority has witnessed more overseas than domestic demand for US dollars under the Convertibility Undertaking, chief executive Joseph Yam Chi-kwong said. According to the undertaking, the authority is ready to convert into US dollars, at a specified exchange rate, any amount of Hong Kong dollars that licensed banks have in clearing accounts maintained with the body. Banks may trade on behalf of their clients, making it difficult to identify the real buyers. Since the introduction of the Convertibility Undertaking in September last year, 33 transactions valued at US$4 billion had been made, Mr Yam said. Only 10 of these transactions occurred in Hong Kong, although the amounts were on average larger. 'I must confess that I am little puzzled about why this should be the case, just as I am curious to know the true identity of those who might be behind these [overseas] transactions,' Mr Yam said on the authority's Web site. 'Could it be that those . . . whose job is to play the market were testing whether the HKMA was there? If this is the case, what then was the intention behind the tests?'