Edward Snowden has been reported to be "healthy and safe" by Julian Assange but his whereabouts remain a mystery as the US hunted the architect of one of the biggest intelligence leaks in history.

The WikiLeaks founder on Monday said Snowden and a WikiLeaks staff member travelling with him, Sarah Harrison, were "healthy and safe and in contact with their legal team".

However, he added that he could not give information on their whereabouts. Snowden reportedly left Hong Kong for Moscow on Sunday, and was said on Monday to have exited on a flight to Cuba. However, journalists travelling on the same plane said his seat was empty.

Strong objections

The White House later said it believed Snowden was still in Russia, said it expected the Russian government to send him back to the US and lodged "strong objections" to Hong Kong and China for letting him go.

A spokesman said on Monday evening that Snowden's exit from Hong Kong "unquestionably'' damaged US-China relations, and that officials believed he was still in Russia and should be handed over.

Snowden has been charged by the US of espionage and spying after revealing to Western newspapers how the US's National Security Agency spies on the internet and phone activities of millions of people. The programme, named Prism, is authorised by a secret court.

US Secretary of State John Kerry said during a visit to India that it would be "deeply troubling" if Moscow defied the US over Snowden, and said the fugitive "places himself above the law, having betrayed his country".

Kerry said that ties with Russia and China could be affected, saying he would be "disappointed" if Snowden was allowed to board a plane. He said the US expected "reciprocity" from Russia because it had transported seven prisoners that Moscow wanted in the past two years.

"I wonder if Mr Snowden chose China and Russia as assistants in his flight from justice because they are such powerful bastions of internet freedom," Kerry said while on a trip to New Delhi.

But the Russian government ignored the appeal and President Vladimir Putin's press secretary denied any knowledge of Snowden's movements.

Asked if Snowden had spoken to the Russian authorities, Peskov said: "Overall, we have no information about him."

He declined comment on the expulsion request but other Russian officials said Moscow had no obligation to cooperate with Washington, after it passed legislation to impose visa bans and asset freezes on Russians accused of violating human rights.

'Grave concern'

Since leaving Hong Kong, where he feared arrest and extradition, Snowden has been searching for a country that can guarantee his security.

Ecuador, which has sheltered Assange, said it was considering a request for asylum from Snowden and that human rights were it main concern.

Ecuador's foreign minister, Ricardo Patino, said: "We will consider the position of the US government and we will take a decision in due course in line with the (Ecuadorean) constitution, the laws, international politics and sovereignty.

The Chinese Foreign Ministry expressed "grave concern" over Snowden's allegations that the US had hacked computers in China. It said it had taken up the issue with Washington.