PRESSURE is building on Hongkong to uphold sanctions against Serbia after the second suspected Serbian ship in less than a week was allowed to enter, load and leave the territory. The Maltese-registered Brisa left Hongkong for Taiwan with general cargo on Sunday, which is seen as further evidence that a fleet of Serbian-controlled ships are sailing around Asia to earn valuable foreign exchange for the war effort. The ship will be safe from arrest as long as it stays in Taiwan, which is not a member of the United Nations, maritime sources said. Hongkong had been tipped off about the Brisa by US intelligence, which listed the 13,000-tonne container ship as Serbian controlled. However, Economic Services Branch officials said they wanted to be sure of the information before arresting a ship and seizing its cargo in powers extended to Hongkong last week. One senior government source said: ''All we really know is that it is registered in Malta. US intelligence is simply not good enough to be used in court in Hongkong, if it came to that.'' Urgent requests have been sent to the Foreign Office in London to back the intelligence to enable the Governor, Mr Chris Patten, to order impoundings. ''Hongkong is a big port and clearly Serbian ships could enter and we must have international obligations to take action,'' the source said. ''We also have obligations to ensure we seize the right ships - we're not going to move until we hear from the Foreign Office.'' But US officials have already backed the intelligence, comprising a list of 55 ships and a ''hit-list'' of about a dozen thought to be active in Asia. Another ship on the list, the Maltese-registered Aire F, was allowed to leave Hongkong for Jakarta last Wednesday. One US diplomat said yesterday the list had been prepared by the Treasury Department and would be used as evidence to seize a ship by the US. ''We stand by this list,'' the diplomat said. The US treated the sanctions against Serbia and Montenegro very seriously, and they expected their allies to treat the UN sanctions seriously and do their best to uphold them also, he said. The US position has been outlined to senior government officials and will be further emphasised in later meetings.