(Baku) Armenia and Azerbaijan accused each other on Monday of violating a new U.S.-brokered ceasefire in fighting over the mountain enclave of Nagorno-Karabakh, casting doubt over the prospects of the latest international push to end a month of clashes.

The third truce in just over two weeks came into force at 8 a.m. local time (0400 GMT). Within minutes, Azerbaijan's defence ministry said in a statement that Armenian forces had shelled villages in the Terter and Lachin regions.

The Nagorno-Karabakh defence ministry denied this and said Azeri forces had launched a missile attack on Armenian military positions on the northeastern side on the line of contact. Armenia's defence ministry said in a statement that the Azeri side violated the ceasefire at around 9.10 a.m. local time.

The latest fighting in Nagorno-Karabakh, a mountainous part of Azerbaijan populated and controlled by ethnic Armenians, erupted on Sept. 27 and is the worst in the South Caucasus since the 1990s. Two Russian-brokered ceasefires have failed to hold.

World powers want to prevent a wider war that might draw in Turkey, which has voiced strong support for Azerbaijan, and Russia, which has a defence pact with Armenia.