The hospital is owned by Ghassan Aboud, a Gulf-based businessman who runs Orient Television.
It was not immediately clear who carried out the attack. Syria is in the throes of civil war and Atmeh is in territory held by rebels fighting to overthrow President Bashar al Assad.
Eyewitness Abdallah Saleh said: "There are 50 casualties and they are being transported to Bab al Hawa [crossing] and to another hospital in Atmeh." He said the hospital was damaged.
The revolt against Mr Assad erupted in March 2011 with protesters calling for more rights and has turned to a militarised uprising under the heavy crackdown of Mr Assad's security forces.
Now, Islamist fighters, including jihadis from across the world, have eclipsed secular groups and are also at war with each other in much of Syria, with the Nusra Front and other Islamists fighting the Islamic State of Iraq and Levant (ISIL) - a group disavowed by al Qaeda.