U.S. Vice President Joe Biden on Monday, April 4 held phone talks with Armenian President Serzh Sargsyan and Azerbaijani leader Ilham Aliyev, the Vice Presidents said in a tweet.

"As I told Presidents Aliyev and Sargsyan, comprehensive settlement in Nagorno Karabakh is critical for their stability, security and prosperity," Biden said.

Meanwhile, Russian Foreign Minister Sergey Lavrov and the U.S. Secretary of State John Kerry discussed the escalation in the Nagorno Karabakh conflict zone during a telephone conversation on Monday, the Russian Foreig Ministry said.

"It was agreed that Russia, the U.S. and France, as the Co-chairs of the OSCE Minsk Group, should intensify efforts for the settlement of the Nagorno Karabakh conflict," the ministry said.

"Also, Lavrov and Kerry condemned the attempts by certain "external players" to instigate a confrontation around Karabakh."

The foreign policy chiefs "expressed serious concern over the escalation in Nagorno Karabakh, reiterating an urgent call for immediate ceasefire," the ministry went on to say.

On the night of April 1-2, Azerbaijani armed forces initiated overt offensive operations in the southern, southeastern and northeastern directions of the line of contact with Nagorno Karabakh.

As many as 20 Karabakh servicemen were killed and 70 were wounded in the course of military operations. In addition, five civilians have been killed, including two children.

26 Karabakh soldiers have gone missing so far.

As of 11:00 p.m., April 4, the Azerbaijani side has lost 29 tanks and over 5 armored vehicles, including an IFV, as well as 1 TOS-1A Solntsepyok multiple rocket launcher, 1 BM-21 Grad multiple rocket launcher, 2 military helicopters and 7 unmanned aerial vehicles.

Seven Karabakh tanks have been neutralized since April 2. The Armenian side has lost eight positions (300 meters overall).