BRUSSELS, BELGIUM - MARCH 23: Armed soldiers guard the entrance to Brussels Central Station after yesterday's terrorist attacks on March 23, 2016 in Brussels, Belgium. Belgium is observing three days of national mourning after 34 people were killed in a twin suicide blast at Zaventem Airport and a further bomb attack at Maelbeek Metro Station. Two brothers are thought to have carried out the airport attack and an international manhunt is underway for a third suspect. The attacks come just days after a key suspect in the Paris attacks, Salah Abdeslam, was captured in Brussels. (Photo by Christopher Furlong/Getty Images)
Passengers leave Brussels airport, on March 22, 2016 in Zaventem , following its evacuation after at least 13 people were killed and 35 injured as twin blasts rocked the main terminal of Brussels airport.AFP PHOTO / JOHN THYS / AFP / JOHN THYS (Photo credit should read JOHN THYS/AFP/Getty Images)
Syrian rebel fighters rest in a field as they hold a position near the frontline on March 1, 2016 in the countryside on the outskirts of the central Syrian city of Homs, as Syria truce enters day four.
The ceasefire applies to Damascus and most of its suburbs, the southern province of Daraa, and parts of Aleppo and Homs provinces but does not apply to territory held by the Islamic State group and Al-Qaeda affiliate Al-Nusra Front.
/ AFP / MAHMOUD TAHA (Photo credit should read MAHMOUD TAHA/AFP/Getty Images)
An Afghan Local Police (ALP) personnel keeps watch during an ongoing battle with Taliban militants in the Marjah district of Helmand Province on December 23, 2015. Military planes have dropped food and ammunition to besieged Afghan forces battling to push Taliban insurgents out of Sangin, officials said December 22, two days after the emboldened militants stormed the opium-growing district. Islamists have captured large swathes of the district in the southern province of Helmand which British and US forces struggled for years to defend.Fleeing local residents reported bloody gunfights as the Taliban advanced on the district centre, highlighting a worsening security situation across Afghanistan a year after NATO formally ended its combat operations. AFP PHOTO / Noor Mohammad / AFP / NOOR MOHAMMAD (Photo credit should read NOOR MOHAMMAD/AFP/Getty Images)
The damaged hospital in which the Medecins Sans Frontieres (MSF) medical charity operated is seen on October 13, 2015 following an air strike in the northern city of Kunduz. Thirty-three people are still missing days after a US air strike on an Afghan hospital, the medical charity has warned, sparking fears the death toll could rise significantly. AFP PHOTO (Photo credit should read STR/AFP/Getty Images)
A general view shows Russian fighter jets on the tarmac at the Russian Hmeimim military base in Latakia province, in the northwest of Syria, on February 16, 2016.
Russia launched air strikes in support of Syria's government in September 2015. / AFP / STRINGER (Photo credit should read STRINGER/AFP/Getty Images)