Vietnam's sudden release of a top dissident who was shuttled from jail to the United States is part of an attempt to secure powerful allies amid a simmering maritime dispute with China, analysts said Wednesday.

Prominent blogger Nguyen Van Hai, whose case had been raised by US President Barack Obama, was freed two years after being sentenced to 12 years in jail for "anti-state propaganda".

Analysts say the timing of his release -- just weeks after Washington partially lifted a 40-year ban on arms sales to Hanoi -- suggests the communist nation is trying to improve ties with its former foe.

Hai's release was seen by Hanoi as necessary "to push forward efforts to build a new strategic partnership with the US", said Jonathan London, of the City University of Hong Kong.

The release of Hai, well-known in Vietnam as a strident anti-China activist, comes as Hanoi is locked in a bitter territorial dispute with Beijing over waters and island chains in the South China Sea.

China placed an oil rig in disputed waters near the Paracel islands in May, sparking deadly anti-Chinese riots in Vietnam.

"There's been aggressive action from China in the East Sea this year, (Vietnam) needs to warm its relations with Europe and the US," Le Quoc Quyet, a Catholic dissident, told AFP, referring to the South China Sea by its Vietnamese name.

Hai's release will both annoy China and "keep the international community happy", said Quyet.

- 'A trick' -

On Wednesday Hai's son Nguyen Tri Dung told AFP he was happy but felt Hanoi had an ulterior motive in releasing his father, better known by his pen name Dieu Cay.

He said it could be connected to the US partially lifting the weapons sales ban and the ongoing negotiations for the Trans Pacific Partnership (TPP) free trade deal.

"It's not the government changing, it is a trick, they do it to get a temporary change in (international) opinion about Vietnam," the son told AFP, speaking in English.

He warned it may set a precedent whereby Hanoi arrests political prisoners and uses them as bargaining chips to "trade for more arms".

Vietnam's Foreign Ministry said Hai was released for "humanitarian reasons", without elaborating.

But if Hanoi was truly serious about improving rights it would abolish laws that allow the arbitrary arrest of dissidents, according to Phil Robertson of Human Rights Watch.

"They're interested in being close to the US, which is connected to China and the TPP," he said, adding that Vietnam was still holding "hundreds" of other political prisoners.

Hai's release follows that of French-trained lawyer Cu Huy Ha Vu, the son of a Vietnamese revolutionary leader, who was freed in April and also went straight to the United States.

"The most plausible explanation for (the releases) is to meet US demands linking an improvement in human rights with the partial lifting of arms sale restrictions on Vietnam," said Vietnam expert Carl Thayer.

US embassy spokeswoman Lisa Wishman said while the US welcomed Hai's release they "remain deeply concerned for all political prisoners in Vietnam".

Hai, who went on hunger strike at least twice to protest at his treatment in jail, had been in detention since September 2008, after first being sentenced to two-and-a-half years for tax fraud.