Russian President Vladimir Putin said Thursday that almost 5,000 people had died in more than three weeks of fighting between Armenian forces and Azerbaijan's army over the disputed Nagorno-Karabakh region.

"According to our data, the number of deaths on both sides is almost 2,000, that is, the total number of deaths is already nearing 5,000," Putin said in a televised appearance. 

Fighting broke out between the Armenian separatists who control the Karabakh province in Azerbaijan and the military late last month and the most recent officially confirmed death toll remains below one thousand.

Both sides have said they are determined to continue fighting and have mostly ignored international efforts to broker a ceasefire.

Two truces brokered earlier this month fell apart almost immediately with both sides accusing the other of violating the agreements to pause fighting.

The foreign minsters of Azerbaijan and Armenia are to meet separately with US Secretary of State Mike Pompeo in Washington on Friday.

Putin said Thursday that he was in "constant" communication with Armenian Prime Minister Nikol Pashinyan and Azerbaijan's President Ilham Aliyev.

"I talk to them on the phone several times a day," Putin said during an online address to members of the Valdai Discussion Club.

Aliyev said earlier Thursday that Azerbaijani forces had taken full control of its border with neighbouring Iran.

He has repeatedly boasted of military gains since fighting erupted September 27 and on Wednesday Armenian Prime Minister Nikol Pashinyan acknowledged his country was facing "a difficult situation" on the frontline.