REUTERS IS UNABLE TO INDEPENDENTLY VERIFY THE CONTENT OF THIS VIDEO WHICH HAS BEEN OBTAINED FROM A SOCIAL MEDIA WEBSITE.
Amateur video purports to show an attack in the Syrian town of Khan Sheikhoun.
The footage was uploaded to a social media website and could not be independently verified by Reuters.
Activists say at least 21 people were killed.
Rebels and pro-government forces blamed each other for the bloodshed.
One rebel fighter says Syrian troops opened fire on a funeral procession.
SOUNDBITE: MEMBER OF FREE SYRIAN ARMY REBELS SAYING (Arabic):
"I'm Lieutenant Ala'a Katainy, the head of the Waleed al-Nisr Martyrs bridge in Khan Sheikhoun. On May 15th and during the visit of the international monitors to Khan Sheikhoun town, security forces and Shabiha militia opened fire on a funeral procession for one of our martyrs, and we destroyed one of the regime's tanks."
Four members of a United Nations monitoring team were caught in the battle and said they were in rebel hands.
One of the monitors contacted by phone did not reply when asked if they were being held prisoner.
A rebel spokesman said a safe exit was being sought for them.
