Syrian activists say that Islamic State extremists have seized almost full control of the ancient town of Palmyra, though it remained unclear how close to the famed archaeological site they have come.

The Syrian Observatory for human rights reported that government forces collapsed in the face of IS attacks and withdrew from the town late on Wednesday.

A media collective for Palmyra also says that IS was now in control of most of the town.

It's not immediately clear how close the militants are to the ruins, which are just southwest of the town.