Reyhanli, Turkey(CNN) Mazin Yusif, 13, sauntered around the hospital with the confidence of a regular, taking us from room to room, introducing the people in each ward.

He arrived here at the Reyhanli State Hospital 24 hours ago, one of around 30 Syrians rushed to this southern Turkish town near the Syrian border after what appears to have been a chemical attack by Syrian warplanes on rebel-held Khan Shaikhoun in Idlib province.

The attack killed at least 70 people, including children, one of the deadliest since the Syrian war began six years ago.

The World Health Organization said victims bore the signs of exposure to nerve agents. Syrian President Bashar al-Assad's military categorically denied using chemical weapons and blamed "terrorist" groups for the carnage.

Mazin wore new clothing, crisp blue jeans and a red, white and blue jacket -- he'd lost his own in the attack. His eyes were bloodshot, his voice was hoarse.

He first introduced his grandmother, 55-year-old Aisha Al-Tilawi, who lay on a bed with an oxygen mask on her mouth, her chest rising and falling quickly.

Aisha Al-Tilawi, 55 years old, Mazin's grandmother.

When asked what had happened, she was almost matter-of-fact. "Around 6 in the morning, the plane struck," she recalled. "Entire families were killed."

She said she saw blue and yellow after the bomb dropped near her home. "We started choking, felt dizzy, then fainted. Mazin was trying to wake up his grandfather," she said.

After some urging, Mazin told us his story.

"At 6:30 in the morning there was an air strike," he said, in the same way others might tell you it was cold outside. Mazin was 7 years old when the uprising in Syria began, and had probably seen more than his fair share of air strikes.

"I saw the explosion in front of my grandfather's house. I ran to their house barefoot, I saw my grandfather sitting ... suffocated," he said, putting his head to the side and rolling his eyes up. "Then I became dizzy."

"When I woke up, I found myself in bed, without clothing," he said, his expression reflecting the confusion he must have felt waking up naked in a strange place.

"I thought I was in the clinic in Khan Shaikhoun. I turned to the guy in the bed next to me and said 'We need to get out of here. The planes will hit us again.'"

Suspected chemical attack in Syria This photo, provided Tuesday, April 4, by the activist Idlib Media Center, shows dead children after a suspected chemical attack in the rebel-held city of Khan Sheikhoun, Syria. Dozens of people were killed, according to multiple activist groups. A man carries a child at a makeshift hospital in Khan Sheikhoun. Activists have blamed the Syrian regime for the attack. In an apparent reference to the attack, two state-run Syrian media outlets reported that there was an explosion at a "rebel poison gas factory" in the countryside of Idlib province. The government has denied using chemical weapons in the past. A child's body is seen in Khan Sheikhoun. In this photo provided by the Idlib Media Center, doctors treat a child after the suspected attack. A baby's dead body is covered. Many of the casualties came as a result of asphyxiation, local doctors say. A dead baby is seen in the aftermath of the attack. "I've never seen anything like it, beyond description," said Fares al-Jundi, a doctor who spoke with CNN. He lives in a nearby village and rushed to the hospital after the airstrikes. Al-Jundi described how whole families were killed: They died of asphyxiation, and foam covered their mouths. Many died suddenly. "I believe this horrible memory will stay with me for the rest of my life," al-Jundi said. An unconscious Syrian child receives treatment at a hospital in Khan Sheikhoun. This photo provided by the Idlib Media Center shows victims of the suspected attack. Civil defense workers try to reduce the effects of toxic gas as they carry out search-and-rescue missions in Khan Sheikhoun.

Almost every hospital in rebel-held Syria has been hit by regime war planes, many multiple times. In other war zones hospitals are off limits for air strikes. Not in Syria.

It was neighbors from Khan Shaikhoun that told Mazin he was in Reyhanli, across the border in Turkey. "They told me my grandfather had died, and my cousins, and their children."

He then listed his relatives who were killed in Tuesday's attack: Yasir, his newly married cousin; another cousin, Ahmed, and his wife and their twin boys. And on and on. Tears welled up in his eyes, but he carried on talking.

In all, he said, 19 of his relatives were killed Tuesday morning.

A Turkish lab for detecting nuclear, biological and chemical weapons is seen crossing the border.

When Mazin said that devastating number, his voice cracked. He lost his struggle to maintain self-control.

His face contorted, his red eyes filled with tears. He plopped down sobbing on the plastic chair in the hospital corridor.

Mazin is only 13 years old. He is a child. And this is his world.