"Across the street I saw my friend Shafiq, 13 years old, outside standing alone," it quoted him as saying. "An armed man in military uniform grabbed him and put him at the corner of a house. He took his own weapon and shot him in the head.

"His mother and big sister – I think she was 14 years old – went outside and started shouting and crying. The same man shot at both of them more than once."

Meanwhile government forces shelled the city of Hama, north of Houla, until 5am on Monday morning after the Free Syrian Army there launched a series of raids on army checkpoints.

It was one of the heaviest barrages against the city since the start of the uprising.

Artillery smashed civilian neighbourhoods where armed opposition groups had been hiding. Snipers infiltrated peripheral buildings using vantage points on rooftops.

"The army has surrounded the districts. Families tried to flee but no one is allowed to leave. They are killing anyone that is moving on the street," said Samer al-Hussain from the Hama News Agency.

An activist who called himself Abu Adnan al-Hamwi, speaking from one of the besieged areas, said: "The sounds of explosions did not stop all day, and as a result of the violent and arbitrary shelling, several houses were destroyed with residents still inside."

Dozens of civilians were rushed to makeshift field clinics, but with few doctors and almost no medicine there was little that could be done. Video footage posted online by activists showed rooms filled with casualties.

The mutilated bodies of men, women and children lay on bloodstained floors, some dead, some still alive as doctors looked on helplessly.

"They can only do basic first aid and try to stop the bleeding," 'Abu Adnan' said. "There is almost no equipment, or medicines. There are no painkillers."

Government hospitals in Hama remained open, but residents refused to take their wounded there for fear they would be deemed rebels and killed.

"The regime is not distinguishing between Free Syrian Army and civilians.

All of Hama has protested against the government, so now we are all enemies," said Mousab al Hamadee from Hama Local Coordinating Committee, By early morning on Monday the guns had fallen silent and the shelling stopped. Smoke rose over the smashed districts and troops stormed the neighborhood of al-Arbeen, setting alight to homes and arresting the remaining residents, activists said.

In other areas, where guerrilla groups affiliated with the Free Syrian Army are still hiding, residents dug holes in the ground to bury their dead, and prepared for the next onslaught.

Further Information:

• SYRIA: Weekend slaughter of 32 children (28 May 2012)
• SYRIA: Over 34 children killed in Syria since truce (2 May 2012)
• SYRIA: Nine from one family killed in bombardment (1 May 2012)
• SYRIA: In Cold Blood - Summary Executions by Syrian Security Forces and Pro-Government Militias (Human Righta Watch, April 2012) 
• SYRIA: 'Stop women and children leaving the country' (20 April 2012) 
• SYRIA: Rebels accused of using child soldiers (29 March 2012) 
• SYRIA: Authorities systematically targeting children (28 March 2012) 
• SYRIA: Villagers 'forced into human shield for Assad's soldiers' (25 March 2012)
• SYRIA: Whole city besieged to "wipe out pockets of armed terrorists” (22 February 2012) 
• SYRIA: SRSG Coomaraswamy calls to immediately halt all violations against children (9 February 2012) 
• SYRIA: Children severely tortured in detention (3 February 2012) 
• SYRIA: Remembering child victims of military crackdown (17 January 2012)
• SYRIA: 6,200 killed, including 400 children (9 January 2012) 
• SYRIA: Boy, 10, slain in home by sniper (12 December 2011) 
• SYRIA: Assad denies responsibility for the killing of thousands of anti-government protesters (7 December 2011)
• SYRIA: Children's rights violations feature in Special Session on Syria (2 December 2011) 
• SYRIA: UN rights panel voices alarm at reported torture of children (25 November 2011)
• SYRIA: Wave of violence targets children (23 November 2011) 
• SYRIA: Three children among latest killed (28 October 2011) 
• SYRIA: Concerns over “rampant torture” (7 October 2011)
• SYRIA: New report indicates over 5,000 deaths since March, including 148 children (Avvaz, 22 September 2011)
• SYRIA: Death toll tops 2,700, including 100 children (20 September 2011) 
• SYRIA: Boy, 12, shot dead during funeral procession (13 September 2011) 
• SYRIA: Teen killed as protesters call for international protection (9 September 2011) 
• SYRIA: Amnesty report shows 88 civilians killed in detention, including 10 children (31 August 2011) 
• SYRIA: A child's killing freeze-frames tragedy (17 August 2011) 
• SYRIA: Defectors describe orders to shoot unarmed protesters (18 July 2011) 
• SYRIA: Second teenager tortured to death (16 June 2011) 
• SYRIA: Soldier 'ordered to fire' on peaceful protesters (10 June 2011)
• More on children's rights in Syria