A joint statement issued by Hanoi at the end of that trip, for example, referred to the two sides underscoring the importance of 'securing peace and stability in the South China Sea'. It also quoted Vietnam welcoming Myanmar's moves to possibly join the Mekong River Commission.

The statement will not go unnoticed in Beijing, which is watching Hanoi's strategic diplomacy closely.

'There has always been the suggestion that Myanmar, at least in part, has long done Beijing's bidding within [the Association of Southeast Asian Nations],' said Dr Ian Storey, a security scholar at Singapore's Institute of Southeast Asian Studies. 'That picture is now much more complex, as the Myanmar-Vietnam relationship shows. Both countries need good relations with China given their long borders, but they also want to expand other regional and international relationships.'

Regional analysts have been struck by the strength of Myanmar's tilt away from China, the former ruling junta's key military patron, in recent months towards a more broadly focused foreign policy, including its recent courtship with Washington. The US maintains sanctions against Myanmar, despite recent political and social reforms.

Thein Sein highlighted the shift in September when he postponed construction of a Chinese-backed dam on the northern reaches of the Irrawaddy River amid mounting public concern at the project's impact. The bulk of the dam's electricity was to be wired to Yunnan province.

The Myanmese president, a former general who rules as a nominally civilian leader, will visit Hanoi with a military delegation. The mission follows the visit in November by Commander-in-Chief General Min Aung Hlaing, who unusually travelled to Hanoi first after being appointed, rather than to Beijing.

Vietnam, too, has shown an increasing willingness to stand up to China on the regional stage, despite attempts to deepen its ongoing fraternal relationship with Beijing.

While Communist Party-ruled Hanoi has been publicly supportive of Myanmar's democratic reforms and closer regional engagement, some analysts wonder whether its leaders would be troubled by the emergence of a new, strong democracy within Southeast Asia.

'Right now, they seem to be happy with a Myanmar that is engaging with Asean, the region and the West,' said one veteran envoy. 'In many respects, that mirrors their own approach and it certainly complicates the regional picture for Beijing.'

Kurt Campbell, US assistant secretary of state for East Asia, acknowledged last month the role that Hanoi had played in nudging Washington and Naypyidaw closer together. While he said it was not clear how Myanmar's changes would affect Vietnam's internal 'dynamics', he said Hanoi had been 'very encouraging in the past about our engagement strategy with Myanmar'.

'There were periods last year when we had made remarkably little success in our engagement ... [and] it was Vietnam who quietly urged us to continue to talk directly to the government, and who provided some interesting and useful information on the way forward,' Campbell said.