Greece has been added to the US Visa Waiver Program, allowing its citizens to travel to the United States for 90 days at a time without a visa, the US Homeland Security Department said Tuesday.

The announcement from Homeland Security Secretary Janet Napolitano, which came as Greek Prime Minister George Papandreou visited Washington, adds Greece to a list of 35 countries whose citizens can visit the United States visa-free.

The waiver will go into effect in a month, and comes after two years of work between US and Greek authorities to strengthen Greece's airport security standards, Napolitano said.

"Our efforts to guard against terrorism while enhancing legal travel and trade depend upon close collaboration with our international partners," she said in a statement.

"I commend our partners in Greece for committing to strong screening and security standards and enhanced information sharing for travel by Greek citizens to the United States as we work together to protect our citizens and strengthen our economies."

The Visa Waiver Program, first established in 1986, allows citizens of designated countries to apply online for an Electronic System Travel Authorization rather than for a visa.