A Yemeni soldier searches a car near the airport in the capital, Sanaa. The United States has ordered Americans to leave Yemen immediately amid a warning of a possible attack. Mohammed Huwais/AFP/Getty Images hide caption

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A Yemeni soldier stands guard Tuesday near Sanaa International Airport. Mohammed Huwais /AFP/Getty Images hide caption

itoggle caption Mohammed Huwais /AFP/Getty Images

An army trooper sits beside a machine gun that is mounted on a patrol vehicle at a checkpoint in Sanaa, Yemen. Security is tight in the capital amid warnings about possible terrorist attacks. Khaled Abdullah /Reuters /Landov hide caption

itoggle caption Khaled Abdullah /Reuters /Landov

John Brennan, President Obama's nominee to be the next CIA director, worked closely with Saudi Arabia to set up a secret U.S. drone base there, The New York Times reported. Brennan's confirmation hearing is Thursday. Kevin Dietsch /UPI /Landov hide caption

itoggle caption Kevin Dietsch /UPI /Landov

Security guards were deployed outside the graffiti-covered walls of the U.S. Embassy in Cairo, Egypt, which came under attack Tuesday. Mohamed Abd El Ghany/Reuters/Landov hide caption

itoggle caption Mohamed Abd El Ghany/Reuters/Landov

This 2010 image, provided by IntelCenter, shows Humam Khalil Abu-Mulal al-Balawi in a posthumous video message posted on extremist websites. The al-Qaida double agent killed seven CIA operatives, a Jordanian spy and himself when he set off a bomb strapped to his body at a base in Afghanistan in December 2009. AP/IntelCenter hide caption

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Outgoing Yemeni President Ali Abdullah Saleh speaks to the press at the presidential palace in Sanaa on Jan. 22. AFP/Getty Images hide caption

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Protesting Yemeni women burn their veils, during a demonstration demanding the resignation of Yemeni President Ali Abdullah Saleh in Sanaa, Yemen.

Hani Mohammed/AP hide caption

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