In this photo released Thursday by a website run by Islamic State militants, damaged Syrian helicopters sit at Palmyra air base, which was captured by ISIS after a battle with the Syrian government forces earlier this week. Uncredited/AP hide caption

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A photo released on Sunday by the Syrian official news agency SANA shows a general view of the ancient Roman city of Palmyra, northeast of Damascus, Syria. Fears have intensified that the self-declared Islamic State, which captured the city on Wednesday, might raze the ruins. AP hide caption

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Smoke rises after a Syrian Rocket launcher shell on Islamic State positions in the ancient oasis city of Palmyra, about 215 kilometers northeast of Damascus on Tuesday. News reports say the Islamic State has taken control of the city. EPA /Landov hide caption

itoggle caption EPA /Landov

Columns in the courtyard of the temple of Baal in the ancient city of Palmyra are damaged by the shelling in this March 14, 2014, photograph. Joseph Eid/AFP/Getty Images hide caption

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People gather around a helicopter reportedly belonging to Syrian government forces that crashed in March in Jabal al-Zawiya in northwest Syria. Islamist rebels captured four crew members, while a fifth was killed, according to the Syrian Observatory for Human Rights. Opposition fighters have made a number of advances in recent weeks. Ghaith Omran/AFP/Getty Images hide caption

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Hamudeh al-Masaadi plays on the shores of Lake Constance near Friedrichshafen, where they wait as their request for asylum is processed. Joanna Kakissis for NPR hide caption

itoggle caption Joanna Kakissis for NPR

Assyrian Christians, who fled their homes because of attacks by the Islamic State, pray at a church on the outskirts of Damascus on March 1. Many minorities in Syria have tried to remain neutral in the conflict, but they often get caught up in the crossfire or are targeted by extremists. Louai Beshara/AFP/Getty Images hide caption

itoggle caption Louai Beshara/AFP/Getty Images

A photo taken in January 2014 and released by the United Nations Relief and Works Agency for Palestine Refugees (UNRWA), shows residents of the besieged Palestinian camp of Yarmouk, queuing to receive food supplies, in Damascus, Syria. AP hide caption

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Secretary of State John Kerry told CBS News the U.S. is pushing for Syrian President Bashar Assad to seriously discuss a transition strategy to quell the Arab country's four-year civil war. Brian Snyder/AP hide caption

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Girls carrying school bags provided by UNICEF walk past destroyed buildings on their way home from school on March 7 in the rebel-held al-Shaar neighborhood of Aleppo, Syria. So many people have fled the city and so much of its infrastructure has been destroyed that nighttime satellite images show 97 percent less light there compared with four years ago. Zein al-Rifai/AFP/Getty Images hide caption

itoggle caption Zein al-Rifai/AFP/Getty Images

Dura Europos, a Roman walled city in eastern Syria, dates back to 330 B.C. The main gate is shown here in a photo from 2010. It's one of the many important archaeological sites militants of the self-styled Islamic State have ransacked and damaged. EPA /Deir Ezz-Zour Antiquities Department/Landov hide caption

itoggle caption EPA /Deir Ezz-Zour Antiquities Department/Landov