US fugitive Edward Snowden, who was granted asylum by Moscow after revealing the extent of US global surveillance, has been reunited with his girlfriend in Russia, his lawyer said on Saturday.

Snowden's longtime partner, US dancer Lindsay Mills, joined him in Moscow in July, it emerged on Friday.

"Love is love," said Snowden's lawyer, Anatoly Kucherena. "She lives with him when she comes here. Moral support is very important for Edward."

He said Mills does not live in Russia permanently because of visa constraints, but visits frequently.

The couple, who previously lived in Hawaii, have been busy exploring Russia, he added. "They go to theaters and cultural events together."

Kucherena spoke after a documentary by Laura Poitras, shown at advance screenings in New York late on Friday, revealed Mills had joined Snowden in Moscow over the summer.

The two-hour documentary - due for release on Oct 24 - paints an intimate and sympathetic portrait of Snowden, holed up in a Hong Kong hotel in June 2013 as he blows the whistle on US National Security Agency dealings and then plots his escape.

Mills is shown preparing dinner with Snowden in a wood-paneled kitchen, but the couple's conversation is not recorded and she is not interviewed.'

'Super hero'

The whereabouts of Mills, who once referred to herself on a blog as "a world-traveling, pole-dancing super hero", were not previously known.

She has never spoken publicly about her boyfriend, who has been charged in the United States with espionage, and some reports had suggested that the couple had split.

But the documentary Citizenfour hints at a much-closer union.

It shows Snowden breaking the news online to Mills from Hong Kong that he would not be back anytime soon after she told him agents had turned up at their home in Hawaii, suspicious about his absence from work.

"It's a tough situation," he says, close to tears on his hotel bed after telling Mills that he "may not be coming back".

"She has no idea what I'm doing," he says. He says he feels "badly" about simply disappearing while she was on vacation, but said he did it to protect her.

After the NSA surveillance story breaks, the couple are shown to be still in touch.

Mills tells him that his rent checks are no longer going through, which he calls "strange" because they had been set up automatically, and that trucks are all over the street outside their home.