Residents inspect blood stains after what activists said was shelling by warplanes loyal to Syria's president Bashar Al-Assad on Saad Al-Ansari school in Aleppo's rebel-controlled Al-Mashad neighbourhood April 12, 2015. REUTERS/Abdalrhman Ismail

Girls who survived what activists said was a ground-to-ground missile attack by forces of Syria's President Bashar al-Assad, hold hands at Aleppo's Bab al-Hadeed district April 7, 2015. REUTERS/Abdalrhman Ismail TPX IMAGES OF THE DAY

Civil defense members put out a fire amid damage in a site hit by what activists said was a barrel bomb dropped by warplanes loyal to Syria's President Bashar al-Assad in Aleppo, April 18, 2015. REUTERS/Hosam Katan 

A man reacts as he looks for survivors at a site hit by what activists said were barrel bombs dropped by forces of Syria's President Bashar al-Assad, at al-Thawra neighborhood in Idlib city April 20, 2015. REUTERS/Ammar Abdullah 

A woman reacts at a site hit by what activists said were an airstrike by forced loyal to Syria's President Bashar al-Assad in Aleppo, April 17, 2015. REUTERS/Hosam Katan 

ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH
An injured man walks amid debris as residents reacts at a site hit by what activists said were barrel bombs dropped by forces of Syria's President Bashar al-Assad, at al-Thawra neighborhood in Idlib city April 20, 2015. REUTERS/Ammar Abdullah TPX IMAGES OF THE DAY

A man inspects the damage from what activists said was due to shelling by warplanes loyal to Syria's president Bashar Al-Assad inside a vegetable market in the al-Maadi neighbourhood of Aleppo April 11, 2015. REUTERS/Rami Zayat 

A general view shows damaged buildings in Aleppo city at night April 11, 2015. REUTERS/Rami Zayat 

Residents wait in line to receive food aid in Yarmouk camp April 14, 2015. A bout of fighting between militants over control of the Yarmouk refugee camp on the edge of Damascus has only compounded the misery of residents already suffering from acute shortages of food, clean water and power. Picture taken April 14, 2015. REUTERS/Moayad Zaghmout 

A man searches for belongings in a damaged car at a site hit by what activists said were barrel bombs dropped by forces of Syria's President Bashar al-Assad, at al-Thawra neighborhood in Idlib city April 20, 2015. REUTERS/Ammar Abdullah 

A woman holds the hands of a girl who survived what activists said was a surface-to-surface missile attack by forces of Syria's President Bashar al-Assad at Aleppo's Bab al-Hadeed district April 7, 2015. REUTERS/Abdalrhman Ismail 

ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH

A boy reacts over his father's dead body as his mother mourns, at a site hit by what activists said were barrel bombs dropped by forces loyal to Syria's President Bashar Al-Assad, in Bab al-Hadid district of Aleppo April 17, 2015. REUTERS/Abdalrhman Ismail 

Children walk beside a painted wall inside Jarmaq school in Yarmouk camp April 14, 2015. The text on the wall reads in Arabic "It's my right to learn." Picture taken April 14, 2015. REUTERS/Moayad Zaghmout 

A man walks on rubble at a site hit by what activists said was a barrel bomb dropped by forces loyal to Syria's President Bashar Al-Assad in Qastal Harami in the old city of Aleppo April 15, 2015. REUTERS/Abdalrhman Ismail 

A man holds two young girls reacting as they rush out from a site hit by what activists said were barrel bombs dropped by forces of Syria's President Bashar al-Assad, at al-Thawra neighborhood in Idlib city April 20, 2015. REUTERS/Ammar Abdullah 

A man rides a motorcycle at a site hit by what activists said was an airstrike by forces loyal to Syria's President Bashar al-Assad in Idlib city April 14, 2015. REUTERS/Mohamad Bayoush 

Residents help a woman amidst the rubble of her house after her family died at a site hit by what activists said was a barrel bomb dropped by warplanes operated by forces of Syria's President Bashar al-Assad in the Al-Sakhour neighbourhood of Aleppo, April 19, 2015. REUTERS/Hosam Katan 

A civil defence member tries to put out a fire inside a damaged building after what activists said was a barrel bomb dropped by forces loyal to Syria's president Bashar Al-Assad in Idlib, after Islamist rebel fighters took control of the area, April 11, 2015. REUTERS/Ammar Abdullah 

A man (R) reacts as he sits on rubble of damaged buildings at a site hit by what activists said was an airstrike by forces loyal to Syria's President Bashar al-Assad in Aleppo's al-Fardous district April 14, 2015. REUTERS/Rami Zayat 

Residents wait in line to receive food aid in Yarmouk camp April 14, 2015. A bout of fighting between militants over control of the Yarmouk refugee camp on the edge of Damascus has only compounded the misery of residents already suffering from acute shortages of food, clean water and power. Picture taken April 14, 2015. REUTERS/Moayad Zaghmout 

Residents inspect the damage from what activists said was due to shelling by warplanes loyal to Syria's president Bashar Al-Assad inside a vegetable market in the al-Maadi neighbourhood of Aleppo April 11, 2015. REUTERS/Rami Zayat 

Residents pack their belongings inside a pick-up truck before fleeing Zabdin village fearing shelling by forces loyal to Syria's president Bashar Al-Assad on the village, Damascus suburb April 12, 2015. REUTERS/Amer Almohibany 

ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH
Residents monitor an injured child after what activists said were airstrikes by warplanes loyal to Syria's President Bashar al-Assad in Da'el town, north of Daraa, April 19, 2015. REUTERS/Alaa Al-Faqir 

A man removes debris in a site hit by what activists said were barrel bombs dropped by forces of Syria's President Bashar al-Assad, at al-Thawra neighborhood in Idlib city April 20, 2015. REUTERS/Ammar Abdullah 

A woman reacts while civil defence members carry her dead child after what activists said was shelling by warplanes loyal to Syria's president Bashar Al-Assad in Aleppo's rebel-controlled Bab Al-Nairab district April 12, 2015. REUTERS/Abdalrhman Ismail

A man inspects a damaged site hit by what activists said were an airstrike by forces loyal to Syria's President Bashar al-Assad in Aleppo, April 17, 2015. REUTERS/Hosam Katan 

A man gestures as he shouts for gathered people to dismiss after what activists said were an airstrike by forces loyal to Syria's President Bashar al-Assad in the village of Sarmin in Idlib province April 15, 2015. REUTERS/Mohamad Bayoush