Russian President Vladimir Putin's spokesperson Dmitry Peskov repeatedly refused to rule out on Tuesday any potential use of nuclear weapons, maintaining they could be considered as a response to an alleged "existential threat."

When asked about what conditions would warrant such a response, Peskov said "if it is an existential threat for our country, then it can be." He did not elaborate further during an interview with CNN International's Christiane Amanpour.

Peskov conceded that Putin has yet to achieve his goals in Ukraine one month after invading, but said the Russian war is "going on strictly in accordance with the plans and the purposes that were established beforehand."

The comments come hours after a senior Pentagon official said Russia has lost more than 10% of forces it sent into Ukraine.

The official, who spoke to reporters on condition of anonymity, said Moscow now has "slightly below" 90% of the forces it amassed in Belarus and western Russia ahead of its military operation that began Feb. 24.

On Monday, a pro-Kremlin tabloid reported what it said was Russian Defense Ministry data indicating 9,861 Russians have been killed and 16,153 injured during the war. The report from Komsomolskaya Pravda was quickly taken down.

The Russian Defense Ministry has not publicly identified the number of casualties that Russia has sustained since early March when it said 498 soldiers had died.

According to UN estimates, at least 953 civilians have been killed and around 1,557 others injured in Ukraine since Russia began its attack on its western neighbor. However, the United Nations has warned that the true toll is likely to be much higher because it has not been able to gain access to areas of heightened hostilities.

The war has also driven 10 million people from their homes, either internally or as refugees who fled abroad, according to the UN refugee agency.