WASHINGTON — Senior Ukrainian officials stepped up intelligence sharing with their American counterparts over the summer as they began to plan the counteroffensive that allowed them to make dramatic gains in the northeast in recent days, a shift that allowed the United States to provide better and more relevant information about Russian weaknesses, according to American officials.
Throughout the war, the United States has provided Ukraine with information on command posts, ammunition depots and other key nodes in the Russian military lines. Such real-time intelligence has allowed the Ukrainians — who U.S. officials acknowledge have played the decisive role in planning and execution — to target Russian forces, kill senior generals and force ammunition supplies to be moved farther from the Russian front lines.
But as Ukraine laid its plans to strike back against the Russians, senior leaders in Kyiv decided that sharing more information with the United States would help secure more assistance, American officials said.
Senior U.S. officials declined to say how much details from the counteroffensive plan Ukraine had shared and how much advice the United States had offered. But one official said Americans had “constantly” discussed with Kyiv ways that Ukraine could blunt the Russian advance in the country’s east.
The gains in the northeast, including the recapture of Izium, a key railway hub, were the most important advances Ukraine has made so far, senior American officials said.
It is not yet clear how much broad strategic importance those gains will have, but there are signs that the current offensive could be the early stages of a drive that could push back the Russian front line significantly, military experts and former intelligence officials said.
“I have thought for several months that Ukraine was going to push Russia back to the 23 February lines by the end of the year,” said retired Lt. Gen. Frederick B. Hodges, a former top U.S. Army commander in Europe, referring to the eve of the Russian invasion. “I watch the Russian logistics, and it just looked to me that they cannot sustain this. Their morale and discipline and all their manpower issues — it’s just not sustainable for them to do what they were trying to do.”
General Hodges said the recent success indicated that Ukraine’s efforts to retake land in the south and east could unfold more quickly than he had previously assessed, even setting the stage for an attempt to retake Crimea next year. Other experts agreed that the tide might be turning for Ukraine.
“The Ukrainian military’s counteroffensive is moving faster and taking terrain even faster than expected,” said Mick Mulroy, a former Pentagon official and C.I.A. officer. “Now is the time for the Ukrainian army to exploit every opportunity they have to degrade and destroy the Russian capacity to fight.”
Current and former U.S. officials praised the sophistication of the Ukrainian preparations for the counteroffensive. The decision by Ukraine to tout its counteroffensive in the south before striking in the northeast is a standard technique for misdirection used by the American Special Operations troops, who have been training the Ukrainians since the annexation of Crimea in 2014.
“These guys have been trained for eight years by Special Ops,” said Evelyn Farkas, the top Pentagon official for Ukraine and Russia in the Obama administration. “They’ve been taught about irregular warfare. They’ve been taught by our intelligence operators about deception and psychological operations.”
Even though messaging around the push in the south may have been something of a feint, officials say that strike also has importance. Even small gains in the south will make it far more difficult for Russian forces to capture the port city of Odesa, a wartime goal of President Vladimir V. Putin.
Nevertheless, current U.S. officials were reserved on Saturday, saying it was too early to determine whether the Ukrainian military could keep up its drive.
The offensive will strain the Ukrainians, who have suffered from shortages in supply, particularly artillery rounds. Their army, too, has taken tough casualties. Going on the offensive is harder and more difficult to maintain than a defense. Some American officials believe the more successful Ukraine is in the next few days, the more Russia will look for ways to strike back.
But the new offensive has demonstrated how the Russian forces have not been able to overcome the fundamental problems laid bare in the opening days of the conflict, American officials said.
The Russian military continues to struggle to get its secure communications to work and to solve its logistics problems. It has also not been able to ramp up its industrial base to meet the demands of the war, multiple officials said. Ukrainian air defenses still threaten Russian aircraft, hampering Moscow from using the full potential of its military.
Russia could revisit its decision not to conduct a large-scale draft to build forces for a renewed push into Ukraine. Mr. Putin could also look to use different tactics in a new phase of the war, especially if he thinks Ukraine’s morale receives a serious boost from the successful counteroffensive. Some senior Ukrainian officials believe the Russians could resort to cyberattacks targeting a broad swath of the country.
Georgii Dubynskyi, Ukraine’s deputy minister of digital transformation, predicted on Friday that in the coming months, as the weather cools, Russia would try again to attack the Ukrainian energy grid, damaging electrical transmission or shutting down pipelines.
“The next phase is, they will try to defeat our energy and financial sectors,” Mr. Dubynskyi said. “We have seen this scenario before.”
Russia attacked Ukraine’s energy grid in 2015. But this time, Ukraine expects Russia to use more targeted phishing attacks to take down parts of the energy grid or combine cyberattacks with a kinetic bombing campaign.