Thomson noted in a blog post he could not say for sure that rebels attacked the hospital, but soldiers at the hospital insisted it was under attack.
Separately, another opposition group reported that a senior scientist and expert in Syria's missile program was killed, along with his wife and son.
Retired Maj. Gen. Nabil Zugheib was assassinated in the Bab Touma district of Damascus, the Syrian Observatory for Human Rights said. There was no immediate claim of responsibility.
Damascus is largely isolated by checkpoints and tanks, witnesses said Saturday. Opposition groups say medicine is running out and residents are appealing for help.
The crisis started in March 2011, when a fierce government crackdown against protesters morphed into a nationwide uprising against the regime.
The United Nations estimates more than 10,000 people have been killed since the crisis began more than 16 months ago. But Rupert Colville, a spokesman for the office of the U.N. high commissioner for human rights, said the United Nations has not been giving out overall death toll numbers since December "because it became impossible to verify the numbers in any meaningful way."
Opposition groups tracking deaths have issued higher tolls. The LCC, for example, estimates more than 16,000 people, mostly civilians, have died.
The Syrian regime has taken a hit with military defections. An official from the Turkish Foreign Ministry said three brigadier generals from Syria arrived in Turkey last week and about two dozen Syrian generals have fled to Turkey.