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JUDY WOODRUFF: New worries over terrorist threats are prompting American officials to take additional safety measures at a number of airports in Europe, the Middle East and Africa that have flights directly to the U.S.

Airports on both sides of the Atlantic ramped up security after American officials warned of potential plots to smuggle bombs on flights to the U.S. They said al-Qaida affiliates in Yemen and Syria are trying to make bombs that current security measures won't detect.

The secretary of homeland security, Jeh Johnson, appeared last night on MSNBC.

JEH JOHNSON, Secretary of Homeland Security: People shouldn't overreact to it or overspeculate about what's going on. But there clearly are concerns centered around aviation security that we need to be vigilant about. There is a terrorist threat to this country that remains.

JUDY WOODRUFF: Al-Qaida's Yemen branch, known as AQAP, was behind the so-called underwear bomber on Christmas Day 2009. A Nigerian man tried and failed to set off plastic explosives inside his underwear on a flight to Detroit.

The following year, AQAP tried again, placing bombs in printer cartridges on cargo planes bound for the U.S. They were intercepted and disarmed. Now the Department of Homeland Security is asking for expanded passenger screening, with special emphasis on smartphones and shoes.

British Prime Minister David Cameron pledged his support today.

DAVID CAMERON, Prime Minister, United Kingdom: This is something we have discussed with the Americans, and what we have done is put in place some extra precautions and extra checks. The safety of the traveling public must come first.

JUDY WOODRUFF: Also, the U.S. Embassy in Uganda warned of a threat to attack Entebbe International Airport tonight. U.S. officials said the information came from Ugandan police.