Any attempt by the US to extradite the NSA whistleblower Edward Snowden from Hong Kong for espionage could take years and be blocked by China, legal experts have said.

The warning comes after it emerged on Friday that the US has charged Snowden with theft of government property, unauthorised communication of national defence information and wilful communication of classified communications intelligence to an unauthorised person. The latter two charges are part of the US Espionage Act.

Legislators in Hong Kong responded by calling for mainland China to intervene in the case. Snowden, 29, who is reportedly in hiding in Hong Kong, was last seen on 10 June. He is understood to have made contact with human rights lawyers in anticipation of a legal action from the US.

The US and Hong Kong have had an extradition treaty since 1998, a year after Hong Kong was transferred from British to Chinese rule. Scores of Americans have been sent back for trial under the treaty.

While espionage and theft of state secrets are not cited specifically in the treaty, equivalent charges could be pressed against Snowden under Hong Kong's official secrets ordinance, legal experts said.

The timeframe for such proceedings remains unclear, but Hectar Pun, a barrister with human rights expertise, said such an extradition could take three to five years.

If Hong Kong authorities did not charge Snowden with an equivalent crime, authorities could not extradite him, lawyers said. In the absence of charges, Snowden was also theoretically free to leave the city, one legal expert said.

Simon Young, a law professor at the University of Hong Kong, said that while the first charge involving theft might readily find equivalence in Hong Kong, the latter two spying offences will likely attract "litigation and dispute" in the courts.

Under Hong Kong's extradition system, a request first goes through diplomatic channels to the government, who decides whether to issue an "authority to proceed". If granted, a magistrate issues a formal warrant for the arrest of Snowden.

Once brought before the court, the judge would decide whether there was sufficient evidence to commit Snowden to trial or dismiss the case, though any decision could be appealed in a higher court.

Snowden could claim political asylum in Hong Kong, arguing he would face torture back home. Article six of the treaty states extradition should be refused for "an offence of a political character".

"The unfairness of his trial at home and his likely treatment in custody" were important factors to consider when assessing Snowden's chances of claiming political immunity from extradition, said Young.

Should a Hong Kong court eventually call for Snowden's extradition, Hong Kong's leader and China could still veto the decision on national security or defence grounds.

Snowden has admitted leaking secrets about classified US surveillance programmes, which he said he did in the public interest. Supporters say he is a whistleblower, while critics call him a criminal and perhaps even a traitor.