Ecuador's decision to study the asylum petition of US intelligence leaker Edward Snowden has angered the opposition and sparked fears that the United States could retaliate by hitting Ecuadoran exports.

The small South American nation has been at the center of the international drama involving the former National Security Agency contractor, whose global flight from American justice has shaken US ties with a clutch of nations including Russia and China.

Business leaders fear that giving Snowden asylum could prompt the United States to take retaliatory measures, with a preferential trade deal set to expire at the end of July unless Washington renews it.

"We don't have the luxury of taking the wrong steps," the head of the Ecuadoran Business Committee, Roberto Aspiazu, told AFP.

"What would we gain from giving political asylum to Snowden -- confirming Ecuador's international image as an anti-imperialist country? I don't think we need that."

The United States is Ecuador's main trade partner, buying 40 percent of the Andean nation's exports, or the equivalent of $9 billion per year.

Foreign Minister Ricardo Patino said Tuesday that the US State Department had contacted his ministry about the case.

"Since this message is only verbal for now, I have asked that they send it to us in writing so that we can take it into consideration when we analyze the asylum request of Mr. Snowden," Patino said during a trip to Vietnam without providing more details.

Snowden was believed to be on his way to Ecuador when he left Hong Kong and landed in Moscow on Sunday, but Russian President Vladimir Putin said he was still in the transit area of the Russian airport on Tuesday, though free to leave.

Ecuadoran President Rafael Correa said he would take a decision on Snowden's application "fully respecting our sovereignty" and his allies in the Congress vowed to back the leftist leader, who has often tussled with the United States in his six years in power.

But the opposition lashed out at the government's involvement in the Snowden affair, accusing Correa of double standards by defending the fugitive who revealed a broad US surveillance program while undermining freedom of the press at home.

"While they want to send Ecuadorans to prison for expressing their opinion, foreigners are given asylum, which confirms the government's double standards," lawmaker Dalo Bucaram told reporters.

Correa granted asylum to WikiLeaks founder Julian Assange in August last year, allowing the Australian buster of US government secrets to take refuge in Ecuador's embassy to London and avoid extradition to Sweden where he faces sex assault claims.