A senior Pentagon official said on Tuesday that some Russian troops in Ukraine have surrendered en masse or sabotaged their own vehicles to avoid fighting due to low morale and fuel and food shortages.

According to the official, some entire Russian units have surrendered without a fight after encountering a surprisingly strong Ukrainian defense. A large number of Russian troops are young conscripts who have been poorly trained and are unprepared for the all-out assault. According to the official, Russian troops have intentionally punched holes in their vehicles’ gas tanks in order to avoid combat in some cases.

However, when these factors are considered together, they may help explain why Russian forces, including an ominous 40-mile convoy of tanks and armored vehicles near Kyiv, Ukraine’s capital, have slowed to a crawl in the last day or two, according to US officials.

The Pentagon official said that, in addition to dealing with fuel, food, and spare parts shortages, Russian commanders leading that armored column toward Kyiv may be “regrouping and rethinking” their battle plans, making adjustments on the fly to gain momentum for what U.S. intelligence and military officials say is an inevitable push to encircle and eventually capture the capital in the coming days.

“They have a lot of power at their disposal,” the Pentagon official said, adding that 80 percent of the more than 150,000 Russian troops stationed on Ukraine’s borders have now joined the fight.

However, US analysts have been struck by the “risk-averse behavior” of such a large force, according to a Pentagon official. Russia launched an amphibious landing to seize Mariupol, a key port city on the Sea of Azov, but forces landed 40 miles away. This gave the Russians more time and space to launch an invasion while also giving the city’s defenders time to prepare.

According to a Pentagon official, Russia’s vaunted air force has yet to gain air superiority over Ukraine, with Russian warplanes being thwarted by Ukrainian fighter jets and a surprisingly resilient and potent array of air defenses ranging from shoulder-fired Stinger antiaircraft missiles to much larger surface-to-air weapons.

Logistics issues have been a stumbling block for Russian forces leaving Belarus, according to a European official on Tuesday.

Prior to the invasion, US and British intelligence raised concerns about the Russian troops’ supply chain in Belarus. According to independent analysts, during military exercises there, some soldiers received insufficient food and fuel supplies. According to the European official, who spoke on the condition of anonymity to discuss operational developments, American officials told allies that the Russians had fixed those problems by mid-February, which was one reason that American warnings about the invasion intensified in the middle of last month.

However, the challenges that Russian forces have faced indicate that their supply chain issues have not been completely resolved, according to the European official.

According to the European official, the logistics failures may help explain the presence of the long, slow-moving convoy of military equipment approaching Kyiv, a tactical failure that presents a key target for the Ukrainian military.

Russian officials, the European official said, expected to have secured air supremacy, at least around Kyiv. But the fact that Ukrainian air defense systems were still operating has put Russian aircraft in danger, as well as the convoy of equipment on the roads.