With anthems, flags and the rare landing of local PLA brass on its flight deck, the Hong Kong visit of the historic USS Carl Vinson aircraft carrier is highlighting the warming of Sino-US military ties. But it also remains a potent symbol of a potentially intractable problem - clear differences of opinion between Beijing and Washington over what naval activities are acceptable under international law. Those differences were at least papered-over on Saturday, as local People's Liberation Army garrison commander, Lieutenant-General Zhang Shibo, flew on a US naval transport plane from Chek Lap Kok with other PLA and local officials and US consul general Stephen Young to land on the Vinson as it steamed towards Hong Kong. The local garrison was also feted with a large cocktail party in one of the carrier's hangars on Monday night, all a far cry from the chill that descended on the relationship last year. PLA brass ignored invitations to board the last carrier to visit, the USS Nimitz, in February - a silent protest at fresh US arms sales to Taiwan. As the cheers and toasts fade, the issue of the different interpretations of the international law of the sea will be one to watch. In short, the US and its allies believe a UN convention allows free passage to all vessels - including military ones - within any country's 200 nautical mile economic zone, a space Washington includes as 'international waters'. And such routine military passage includes, by its nature, surveillance. Beijing officials, both publicly and privately, insist they will respect freedom of navigation but US surveillance is not acceptable near its coasts. It is a long-standing position - but one coming under close scrutiny as China's navy expands and US deployments also increase in response. The region's waters are growing a lot more crowded. The Sino-US strategic and economic dialogue in Washington this month saw law-of-the-sea issues raised and, buried in the closing documents, is a pledge to continue rounds of discussions on the issue. PLA academic analysis in the PLA Daily highlighted the issue. 'These results indicate that it is imperative for China and the US to understand each other's strategic positioning on major international and regional security issues,' wrote Zhao Weibin and Xiao Shizhong of the PLA Academy of Military Sciences. Letting military brass 'directly communicate is conductive [sic] to the pragmatic co-operation ... in international security'. Carl Vinson strike group commander Rear Admiral Samuel Perez acknowledged the problem when asked, upon anchoring in Hong Kong, whether China viewed his ship as a surveillance vessel. 'We understand the claim but we also maintain our rights to operate in international waters ... so I'm sure that we will find a solution.' No one on either side is pretending it is going to be simple. At its core is a Pentagon push to hammer out cold war-era rules-of-the-road at sea of the sort they had with the Soviet Union. Some regional security analysts believe that at some point China will budge as it finds it will want to do to other countries precisely what it objects to the US doing, just as Soviet naval ships routinely sailed close to the US coast. Others note, too, however, that China's weapons build-up is already making it more difficult for carriers to operate in East Asia in a time of conflict - a factor that could mean Beijing is prepared to hang tough on the issue. In the interim, expect the US side to continue to push for carrier visits to Hong Kong beyond the ebb and flow of relations - and to attempt to pour on the charm when they do.