Snowden, U.S. engaged in intercontinental game of cat-and-mouse

(CNN) -- The United States is caught up in an intercontinental game of cat-and-mouse with Edward Snowden, the computer contractor who exposed details of secret U.S. surveillance programs.

As Snowden apparently tries to hop from country to country with help from the anti-secrecy group WikiLeaks, the United States is sternly calling for his return.

Snowden reportedly traveled from Hong Kong to Moscow over the weekend. It wasn't clear where he was Monday, but Wikileaks -- which is helping him stay out of Washington's reach -- said he plans to eventually travel to Ecuador to seek asylum.

At the same time, the U.S. government is attempting to block his path, calling on the countries involved to hand him over. But its clout appears limited, with Snowden expected to travel through a series of nations that have little reason to heed its request.

U.S. Secretary of State John Kerry on Monday urged Russia to turn Snowden over.

"I would urge them to live by the standards of the law because that is in the interest of everybody," he said.

But Russian media have reported that the country's security services say they have no basis for arresting Snowden. Russian officials didn't immediately comment on the matter Monday.

Where's is he?

Hong Kong, where Snowden had been holed up for weeks, allowed him to leave Sunday, the chief executive of the semi-autonomous Chinese territory said Monday.

The Russian Foreign Ministry later said Snowden was in Moscow.

He spent the night at Moscow's Sheremtyevo airport, according to media reports, and was expected to board a flight to Cuba on Monday, Russia's semi-official Interfax reported.

A CNN journalist on a flight to Cuba said Snowden did not appear to be in the cabin, and Interfax later reported that he did not board the plane and may be planning on taking the next flight to Cuba.

Officials at the airport declined Monday to say whether Snowden remained there.

Ecuador 'analyzing' request

It seems unlikely that Cuba, Venezuela or Ecuador -- the nations on Snowden's potential itinerary -- would be inclined to send him back to the United States.

The U.S. government has already asked those three Latin American countries not to admit Snowden or to expel him if they do, a senior Obama administration official told CNN on Sunday.

But Cuba and Venezuela have long had strained relations with Washington. And Ecuador has already given WikiLeaks founder Julian Assange refuge in its embassy in London for a year after he unsuccessfully fought extradition to Sweden in British courts.

Assange say he fears Sweden, which wants him for questioning over sexual assault allegations, would transfer him to the United States.

The Ecuadorian government is "analyzing" an asylum request from Snowden "with a lot of responsibility," Foreign Minister Ricardo Patino said Monday.

"It has to do with the freedom of expression, with the security of citizens around the world, and therefore we have to analyze it deeply," Patino told reporters in Hanoi, Vietnam.

U.S. warns Hong Kong

As U.S. officials try to figure out how to pin Snowden down, they are also voicing displeasure about the Hong Kong government's decision to let him leave Sunday after he spent several weeks there leaking a series of classified documents that embarrassed and angered the Obama administration.

The United States publicly announced Friday that it was seeking to extradite Snowden on charges of espionage and theft of government property.

But the semi-autonomous Chinese territory declined to act on the U.S. request for a provisional arrest warrant, saying it needed more information. Without that information, it said, it had no reason to stop him from getting on the plane to Moscow.

"We have registered our strong objections to the authorities in Hong Kong as well as to the Chinese government through diplomatic channels," Hayden said Monday, "and noted that such behavior is detrimental to U.S.-Hong Kong and U.S.-China bilateral relations."

Officials in Washington have disputed Hong Kong's assertion that the U.S. request didn't fully meet requirements.

"They came back to us with a few questions late Friday and we were in the process of answering those questions," a Justice Department official said Sunday. "We believe we were meeting those requirements."

The surveillance controversy

Snowden has admitted he was the source who leaked classified documents about the National Security Agency's surveillance programs to the British newspaper the Guardian and to The Washington Post. The documents revealed the existence of programs that collect records of domestic telephone calls in the United States and monitor the Internet activity of overseas residents.

Snowden gave up a comfortable life "in order to bring to light what he believed was serious wrongdoing on the part of our political officials," said Glenn Greenwald, the Guardian columnist who co-authored the stories. "And he's now at best going to spend the rest of his life on the run from the most powerful government on Earth."

The disclosures shook the U.S. intelligence community, raising questions about whether the NSA was eroding American civil liberties.

Snowden told the Guardian that he exposed the surveillance programs because they posed a threat to democracy, but administration officials said the programs are vital to preventing terrorist attacks and are overseen by all three branches of government.

"We have not in a single case had a place where a government official engaged in willful effort to circumvent or violate the law. Zero times have we done that," Gen. Keith Alexander, the NSA's director, told ABC's "This Week."

Snowden was a Hawaii-based computer network administrator for Booz Allen Hamilton, an NSA contractor. Alexander said Snowden "betrayed the trust and confidence we had in him" and is "not acting, in my opinion, with noble intent."

Greenwald said Snowden has been extremely judicious about what he has revealed.

"I know that he has in his possession thousands of documents which if published would impose crippling damage on the United States' surveillance capabilities and systems around the world. He has never done any of that," Greenwald told CNN.

An online petition calling on the White House to pardon Snowden passed the key threshold of 100,000 signatures over the weekend.

The petition, which describes Snowden as "a national hero," had more than 110,000 signatures early Monday.

The White House says it will respond to any petition on its site that gathers more than 100,000 signatures in 30 days.

CNN's Alison Harding, Phil Black, Matt Smith, Catherine E. Shoichet, Jill Dougherty, Carol Cratty, Nic Robertson and Alla Eshchenko contributed to this report.