A new ceasefire in the mountain enclave of Nagorno-Karabakh appeared to be in jeopardy Monday, with ethnic Armenian forces and Azerbaijan accusing each other of renewed shelling.

The ceasefire was agreed Saturday after a deal brokered by Russia a week earlier failed to halt the worst fighting in the South Caucasus since the 1990s. More than 1,000 people have been killed since fighting began on Sept. 27.

The failure to halt the fighting has raised fears of a humanitarian crisis and put new strains on ties between Turkey, which strongly backs Azerbaijan, and its allies in NATO, which want the fighting to stop, Reuters reported.

Russia, which has a defense pact with Armenia, could also be at risk of being sucked into a regional war.