NEW YORK, NY - SEPTEMBER 28: (AFP OUT) U.S. President Barack Obama (L) and Russian President Valdimir Putin toast during a luncheon hosted by United Nations Secretary-General Ban Ki-moon during the 70th annual UN General Assembly at the UN headquarters September 28, 2015 in New York City. Obama held a bilateral meeting with Indian Prime Minister Narendra Modi and with have a face-to-face meeting with Putin later in the day. (Photo by Chip Somodevilla/Getty Images)
Russian President Vladimir Putin (C), accompanied by Defence Minister Sergei Shoigu (4thL), meets with military chiefs at the Bocharov Ruchei state residence in Sochi on November 16, 2016. / AFP / SPUTNIK / ALEXEI DRUZHININ (Photo credit should read ALEXEI DRUZHININ/AFP/Getty Images)
Syrians leave a rebel-held area of Aleppo towards the government-held side on December 13, 2016 during an operation by Syrian government forces to retake the embattled city.
UN chief Ban Ki-moon expressed alarm over reports of atrocities against civilians Monday, as the battle for Aleppo entered its final phase with Syrian government forces on the verge of retaking rebel-held areas of the city.
/ AFP / KARAM AL-MASRI (Photo credit should read KARAM AL-MASRI/AFP/Getty Images)
Russia's President Vladimir Putin (R) and ExxonMobil Chairman and CEO Rex Tillerson (L) attend at the ceremony of the signing of an agreement between state-controlled Russian oil company Rosneft and ExxonMobil in the Black Sea port of Tuapse on June 15, 2012. AFP PHOTO/ RIA-NOVOSTI/POOL/MIKHAIL KLIMENTYEV (Photo credit should read MIKHAIL KLIMENTYEV/AFP/Getty Images)
US President Barack Obama speaks alongside US Vice President Joe Biden (2nd R), US Secretary of State John Kerry (R) and Secretary of Homeland Security Jeh Johnson (L) at CIA Headquarters in Langley, Virginia, April 13, 2016, following a meeting with the National Security Council. / AFP / SAUL LOEB (Photo credit should read SAUL LOEB/AFP/Getty Images)
Western special forces, supporting the US-backed Kurdish-Arab forces, deploy on the frontline, one kilometre from the Syrian town of Ain Issa, some 50 kilometres (30 miles) north of Raqa, on November 6, 2016 as they launched an offensive on the Islamic State group's de facto Syrian capital.
The start of the assault by the Syrian Democratic Forces (SDF) came as Iraqi forces fought inside Mosul for the third day running, with the jihadists putting up fierce resistance.
/ AFP / DELIL SOULEIMAN (Photo credit should read DELIL SOULEIMAN/AFP/Getty Images)
A Russian Sukhoi Su-35 bomber lands at the Russian Hmeimim military base in Latakia province, in the northwest of Syria on May 4, 2016.
Syria's conflict erupted in 2011 after anti-government protests were put down. Fighting quickly escalated into a multi-faceted war that has killed more than 270,000 people and forced millions from their homes. / AFP / Vasily Maximov / MOY (Photo credit should read VASILY MAXIMOV/AFP/Getty Images)
Russia's navy ships and helicopters take part in a military exercise called Kavkaz (the Caucasus) 2016 at the coast of the Black Sea in Crimea on September 9, 2016. / AFP / VASILY MAXIMOV (Photo credit should read VASILY MAXIMOV/AFP/Getty Images)
WASHINGTON, DC - NOVEMBER 17: The early morning sun begins to rise behind the U.S. Capitol on November 17, 2016 in Washington, DC. Later today, Vice President-elect Mike Pence is scheduled to meet with Congressional lawmakers on Capitol Hill. (Photo by Mark Wilson/Getty Images)
FAYETTEVILLE, NC - DECEMBER 06: President-elect Donald Trump shakes the hand of retired U.S. Marine General James Mattis after naming him Secretary of Defense at Crown Coliseum on December 6, 2016 in Fayetteville, North Carolina. Trump took time off from selecting the cabinet for his incoming administration to celebrate his victory in the general election. (Photo by Sara D. Davis/Getty Images)
TAMPA, FL - SEPTEMBER 17: U.S. President Barack Obama speaks during a visit to the U.S. Central Command at the MacDill Air Force Base on September 17, 2014 in Tampa, Florida. Obama visited the base to receive a briefing from his top commanders at CENTCOM, on the strategy to degrade and destroy ISIL. (Photo by Joe Raedle/Getty Images)
A picture taken on December 5, 2016 shows destroyed buildings in Aleppo's eastern al-Shaar neighbourhood as Syrian pro-government troops advance towards the area through Karm al-Jabal district during their offensive to retake Syria's second city.
The Syrian Observatory for Human Rights said the advances of the Syrian and allied forces left the large Shaar neighbourhood encircled by government forces and in danger of falling from rebel control. With the capture of Shaar, the army would hold 70 percent of east Aleppo, four years after rebels first seized it. / AFP / GEORGE OURFALIAN (Photo credit should read GEORGE OURFALIAN/AFP/Getty Images)
BERLIN, GERMANY - NOVEMBER 17: U.S. President Barack Obama speaks at a press conference with German Chancellor Angela Merkel at the Chancellery on November 17, 2016 in Berlin, Germany. President Obama is on his last trip to Europe and is scheduled to hold talks with Chancellor Merkel as well as French President Francois Hollande, British Prime Minister Theresa May, Italian Prime Minister Matteo Renzi and Spanish Prime Minister Mariano Rajoy in Berlin tomorrow. (Photo by Carsten Koall/Getty Images)
BEIJING, CHINA - SEPTEMBER 16: Chinese People's Liberation Army navy soldiers of a guard of honor look at Chinese President Xi Jinping (Front) during a welcoming ceremony for King Hamad Bin Isa Al Khalifa of Bahrain outside the Great Hall of People on September 16, 2013 in Beijing, China. At the invitation of Chinese President Xi Jinping, King Hamad Bin Isa Al Khalifa of Bahrain paid a state visit to China from September 14 to 16. (Photo by Feng Li/Getty Images)