Russia's seizure of Mariupol would enable it to go on to create a “land bridge” from eastern Ukraine to Crimea, a senior US defence official said on Tuesday.
But the official said Moscow had not yet won full control of the battered city and that Mariupol remained “contested” despite an intense Russian bombardment for the last six weeks.
On Tuesday, Russia’s Defence Ministry set an almost immediate deadline for Ukrainian soldiers in the city to surrender. But so far the Pentagon has seen no signs of capitulation, the official said.
“We believe the Russians want [Mariupol] for a number of reasons, One is giving them an unencumbered land bridge from the Donbas to Crimea,” the official said.
Russia annexed Ukraine's Crimea region in 2014. A Russian-controlled passage south-west along the Sea of Azov coast from Mariupol to Crimea would also provide Moscow with greater flexibility in its Donbas offensive, the Pentagon said.
Moscow heavily outnumbers Ukrainian forces in Mariupol and across the broader Donbas region, and Ukrainian President Volodymyr Zelenskyy is pleading with the West for more weapons to counter the invaders.
But the defence official said the fall of Mariupol and Russia's capture of the Donbas region were not foregone conclusions.
“We don't see it that way, and we're doing everything we can to make sure that it's not inevitable,” the official said.
The official did not deny Ukrainian and Russian claims that Moscow has begun its offensive in eastern Ukraine, but said operations so far were “limited” in nature.
“We think that this is a prelude of larger offensive operations that are potentially still in the offing here,” the official said.
The Pentagon also said Ukraine's military had received additional aircraft as well parts for repairs to get damaged aircraft flying again, but retracted the claim on Wednesday after the Ukrainian air force said it had not received any additional planes.
Ukraine has defied expectations of allies and military experts by not only keeping its air force operational nearly two months after the start of Russia's invasion but actually repairing aircraft and, apparently, adding to its inventory.
The Pentagon estimates Russia has launched more than 1,670 missiles into Ukraine and has 78 battalion tactical groups in the country, with two added in the last 24 hours.
Regarding US plans to defend Finland against Russian aggression if it joins Nato, the official said: “We do not see an active threat right now to Finland and there's been no request by Finland for any outside support for their defence.”
Last week, former Russian prime minister and deputy chairman of Russia's Security Council, Dmitry Medvedev, gave a warning to Nato that if Sweden and Finland were to join the alliance, Russia would deploy nuclear weapons and hypersonic missiles in the Baltic Sea.
“There can be no more talk of any nuclear-free status for the Baltic,” Mr Medvedev said.
The official said the Pentagon could not independently verify claims that the Ukrainian town of Kreminna had fallen to the Russians.
The official said US deliveries of howitzer artillery systems to Ukraine were now being “prioritised” and “you will begin to see them arrive in the region very, very soon”.
This story was updated on April 20 to reflect Pentagon correction.