The government of Armenia reported this.
"Pashinyan announced Turkey's involvement in hostilities and the transfer of Syrian mercenaries and terrorists to Azerbaijan," reads a message on Twitter.
At the same time, O'Brien confirmed the US interest in an early ceasefire in Karabakh and the establishment of stability in the region.
Earlier, Turkish leader Recep Tayyip Erdogan called the liberation of the "occupied lands of Azerbaijan" a condition for long-term peace in the Caucasus.
Azerbaijani President Ilham Aliyev also stated that hostilities in Nagorno-Karabakh will stop if Armenia completely withdraws its forces from the region.
In turn, Pashinyan stressed that Azerbaijan had launched a "full-scale military aggression" and a war was going on in Karabakh.
The Kremlin called on the parties for an early ceasefire in the region.