American officials say there are indications that Ukraine has started to shift some of its more seasoned combat forces from the east to the south. But even the most experienced units have been reconstituted a number of times after taking heavy casualties. These units rely on a shrinking cadre of senior commanders. Some platoons are mostly staffed by soldiers who have been wounded and returned to fight.

Ukraine has penetrated at least one layer of Russian defenses in the south in recent days and is increasing the pressure, U.S. and Ukrainian officials said. It is close to taking control of Robotyne, a village in the south that is near the next line of Russian defenses. Taking the village, American officials said, would be a good sign.

A spokesman for the Ukrainian military did not respond to text messages or phone calls on Tuesday.

But some analysts say the progress may be too little too late. The fighting is taking place on mostly flat, unforgiving terrain, which favors the defenders. The Russians are battling from concealed positions that Ukrainian soldiers often see only when they are feet away. Hours after Ukrainians clear a field of mines, the Russians sometimes fire another rocket that disperses more of them at the same location.

Under American war doctrine, there is always a main effort to ensure that maximum resources go to a single front, even if supporting forces are fighting in other areas to hedge against failure or spread-out enemy defenses.