In the buildup to his 30th birthday last week, US fugitive Edward Snowden came to the sudden realisation over a dinner of pizza and Pepsi that Hong Kong may be a less welcoming refuge than he'd thought, his legal advisor said on Tuesday.

Even before the United States issued an arrest warrant on Friday -- his birthday -- the truth was dawning on the former IT technician that he risked prolonged detention in Hong Kong with no creature comforts: no computer, and no Internet access.

Snowden's dramatic flight from Hong Kong on Sunday reportedly came about after he received assurances from the local government -- backed by Beijing -- that he would be free to go, provoking a stinging response from irate officials in Washington.

Prominent pro-democracy lawmaker Albert Ho, one of three lawyers who agreed to represent Snowden in Hong Kong, said the lawyers and a shadowy go-between all conveyed similar advice before he fled to Russia and possibly asylum elsewhere.

In an interview with AFP, Ho gave new insight into the claustrophobic mood that was enveloping the one-time contractor for the National Security Agency (NSA) as the long arm of US justice reached out to apprehend him.

On Tuesday last week, the three lawyers gathered at the private apartment of a local supporter who had taken Snowden in. He had checked out of the chic Mira hotel on June 10, three weeks after arriving in Hong Kong with reams of NSA secrets downloaded to his laptops.

"He came to Hong Kong alone. He felt helpless and contacted human rights organisations well connected with lawyers," Ho said.

After leaving the Mira, "he did change (locations) one or two times", Ho said. "But it was done very carefully, at night."

With Snowden's birthday looming, the legal team bought in party food for the Tuesday dinner -- pizza, chicken wings and sausages, washed down with Pepsi.

Fearful of the possibility of hostile surveillance, their client also insisted that everyone's mobile phones be placed in the fridge, Ho said.

It was his first meeting with the lawyers, and they impressed on him that Hong Kong police could well detain him once a US warrant came, and that conditions in the detention centre would be spartan to say the least.

His living space in the safe houses was "tiny" but life was tolerable "as long as had a computer so he could contact those he wanted to", Ho said. Detention would have meant losing his laptops, "and that would have been intolerable to him".

"The crux of the issue was whether he could get bail," the lawmaker said, confirming details first given by the New York Times in an account published late Monday of Snowden's last days in Hong Kong.

Snowden was warned that detention without bail would drag on for months as extradition proceedings played out in the courts. "He became extremely worried," Ho said.

After the Tuesday dinner, according to Ho, an intermediary approached Snowden to relay that the Hong Kong government could not interfere in a circuitous legal process, but would not impede him if he tried to fly out of the airport.

Ho refused to identify the middleman, and Hong Kong's leader on Monday said he was "completely unaware" of any such person.

Chief Executive Leung Chun-ying reiterated the government's line that owing to deficient US paperwork, Hong Kong had no legal basis to prevent Snowden's departure and that officials were scrupulous in following the law.

On Tuesday, China's foreign ministry said White House claims that it took a "deliberate choice" to spring a fugitive were "groundless" and "unreasonable".

Ho said he believed Snowden had firmed up a decision to leave Hong Kong by Friday evening, after news of the US indictment emerged, and after the purported assurances from the mystery middleman in Hong Kong.

At that time, Hong Kong says it was asking the United States for clarification on its arrest warrant to justify detaining Snowden. No word had come through that his passport had been revoked, and the warrant was missing vital data such as Snowden's full name and his passport number, officials in the city say.

In the end, Ho said, Snowden was accompanied to the airport on Sunday by another of the lawyers, Jonathan Man, as insurance if police or immigration officials intervened on behalf of US authorities.

No such intervention came, although the WikiLeaks whistleblowing operation says it also facilitated his departure by providing him with a refugee document issued by Ecuador and its own lawyer to accompany him on the flight to Moscow.

Hong Kong government officials refused to comment on Ho's account.