President Joe Biden will announce new US and allied sanctions on Russia in response to its ongoing war against Ukraine, a senior American official said Tuesday.

National Security Advisor Jake Sullivan told reporters at the White House that Biden will announce the new economic penalties as part of a coordinated action with US partners and will further tighten existing sanctions to crack down on "evasion, and to ensure robust enforcement."

Biden will depart Tuesday for Brussels and participate in a series of meetings beginning March 24, including a NATO summit and European Council meeting, to discuss ongoing western concerns about Russia's war.

As the war nears its one-month mark, Sullivan offered a somber assessment.

"There will be hard days ahead in Ukraine, hardest for the Ukrainian troops on the frontlines and the civilians under Russian bombardment," he said. "This war will not end easily or rapidly."

Still, Sullivan maintained that Russia has "manifestly failed to" achieve it objectives in Ukraine and has so far "achieved the opposite."

"The brave citizens of Ukraine are refusing to submit. They're fighting back. They're defending their homes, they're defending their cities and although Russia may take more territory in these brutal military operations, it will never take the country away from the Ukrainian people," he said.

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 UN 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./aa