▶ Watch Video: Russian troop movements in Ukraine, and what they mean for the war

U.S. officials expect the fighting to become more intense as Russia’s main effort shifts to the Donbas region because more forces will be concentrated in a smaller geographic area.  

The officials consider Mariupol, in southeastern Ukraine, as the linchpin. If and when it falls, the Russians will be on the left flank of the Ukrainian armed forces who have been fighting in the Donbas. Control of Mariupol would give Russia access to a major port, and it would create a land bridge extending from the Donbas, in eastern Ukraine, to Crimea, in the south, allowing Russia to open up its resupply lines.

The Russian defense ministry earlier this week announced it would reprioritize eastern Ukraine over its efforts in the North, specifically against Chernihiv and Kyiv, the capital. 

The Pentagon has seen indications that a small number of Russian forces are moving north from Kyiv, likely to refit and resupply the units for use elsewhere in Ukraine, including potentially the Donbas, Pentagon spokesperson John Kirby said in a briefing this week. 

Since the beginning of the invasion, the Pentagon assessed the goal of some Russian lines of assault against Kharkiv and Mariupol were an attempt to section off the east and trap the Ukrainian troops fighting in the Donbas, so they would not be able to help defend other areas. 

So far, the Russians have been unsuccessful. They have failed to take Kharkiv and have surrounded Mariupol for at least the past week without winning control. A senior U.S. defense official said on Thursday that the Ukrainians are fighting very hard inside Mariupol to keep Russia from a complete takeover. 

Pentagon spokesperson John Kirby said this week that the Russians have become much more active in the Donbas region. There are about 1,000 mercenaries from the Wagner Group dedicated to the Donbas, and the Russians have prioritized airstrikes in the region.  

It remains to be seen whether the refitting and repositioning of troops will make a difference. 

As the battle map shifts, the U.S. continues to ship in military equipment to the Ukrainians. Weapons included in the $800 million security assistance package recently approved by President Biden have started to arrive in the region. 

A U.S. official says switchblade drones are scheduled to arrive Friday. The drones are equipped with a javelin anti-tank warhead to hit armored tanks. The Ukrainians have been effective at using the Stinger anti-aircraft and Javelin anti-tank weapons against the Russians.