SHANNON, Ireland - An ocean away from the United States, travelers flying out of the international airport here on the west coast of Ireland are confronting one of the newest lines of defense in the war on terrorism: the U.S. border.

In a section of this airport carved out for the Department of Homeland Security, passengers are screened for explosives and cleared to enter the United States by U.S. Customs and Border Protection officers before boarding. When they land, the passengers walk straight off the plane into the terminal without going through border checks.

At other foreign airports, including those in Madrid; Panama City, Panama; and Tokyo, U.S. officers advise the local authorities. U.S. programs in other cities expedite travel for passengers regarded as low risk.

The programs reflect the Obama administration's ambitious effort to tighten security in the face of repeated attempts by al-Qaeda and other terrorists to blow up planes headed to the United States from foreign airports.

The thinking is simple: By placing officers in foreign countries and effectively pushing the U.S. border thousands of miles beyond the country's shores, Americans have more control over screening and security. And it is far better to sort out who is on a flight before it takes off than after a catastrophe occurs.

"It's a really big deal - it would be like us saying you can have foreign law enforcement operating in a U.S. facility with all the privileges given to law enforcement, but we are going to do it on your territory and on our rules," the secretary of the Department of Homeland Security, Janet Napolitano, said on a flight back to the United States from the Middle East, where she negotiated with leaders in Israel and Jordan about joint airport security programs. "So you flip it around, and you realize it is a big deal for a country to agree to that. It is also an expensive proposition."

Airports in 14 countries are participating in the programs, which have been expanded over the last several years and have required substantial concessions from foreign leaders. In many cases, they have agreed to allow U.S. officers to be placed in the heart of their airports and to give them the authority to carry weapons, detain passengers, and pull them off flights.

In December, the government of Abu Dhabi signed a letter of intent to build a terminal where U.S. officers will clear passengers to enter the United States, the most ambitious agreement the United States has struck so far with an Arab country. On her recent trip to Jordan, Napolitano began negotiations with the ruling family there about similar efforts.

The Obama administration sped up expansion of the programs, which cost about $115 million a year, after an al-Qaeda operative tried to detonate explosives hidden in his underwear on a flight from Amsterdam to Detroit on Christmas Day in 2009.

The security at foreign airports drew fresh public attention last month after new reports that intelligence agencies thwarted another plot by al-Qaeda to detonate an underwear bomb on a U.S.-bound airliner.