WASHINGTON

Russia is in the midst of a mass redeployment of its forces to "overwhelm" Ukrainian forces in the country's east, the US warned on Monday.

National Security Advisor Jake Sullivan said the Kremlin has revised its "war aims," and is currently attempting "to concentrate its offensive operations in Eastern and parts of southern Ukraine" after largely pulling back from the country's north.

"All indications are that Russia will seek to surround and overwhelm Ukrainian forces in eastern Ukraine," he told reporters at the White House.

Eastern Ukraine is home to the Donbas where two pro-Russia separatist republics were proclaimed, and subsequently recognized by Russian President Vladimir Putin in February.

In a sign of the mass scope of Russia's military shift, Sullivan said Moscow is seeking to redeploy dozens of battalion tactical groups, which include tens of thousands of soldiers. He further warned that the next step of Russia's war would likely be "protracted," later clarifying that could be "months or longer."

"We should be under no illusions that Russia will adjust its tactics, which have included, and will likely continue to include, wanton and brazen attacks on civilian targets," said Sullivan.

"While Moscow may be interested now in using military pressure to find a political settlement, if this offensive in the east proves to gain some traction, Russia could regenerate forces for additional goals, including trying to gain control of yet more territory within Ukraine," he said.

The Russian war against Ukraine, which started on Feb. 24, has been met with international outrage, with the European Union, US, and UK among others implementing sweeping financial sanctions on Moscow.

At least 1,430 civilians have been killed in Ukraine and 2,097 injured, according to UN estimates, with the true figure feared to be far higher.

More than 4.1 million Ukrainians have also fled to other countries, with millions more internally displaced, according to the UN refugee agency.