This story has been updated with additional information from Pentagon officials on the amphibious invasion near Mariupol and ship positions near Odesa.
Thousands of Russian naval infantry have joined with other Russian ground forces and are about 30 miles to the southwest of the Ukrainian port city of Mariupol, U.S. defense officials said on Sunday.
The amphibious forces came from a Friday landing into Ukraine, through the Sea of Azov, and have advanced 12 miles east toward Mariupol, the senior defense official told reporters on Sunday.
“Our assessment is that Mariupol is defended and the Ukrainians will put up a resistance there,” a defense official said.
On Saturday, USNI News contributor H I Sutton published satellite photos showing a patrol boat, Russian Black Sea Fleet flagship RTS Moskva (121) and a Russian surveillance ship were operating near Zmiinyi Island.
Pentagon Press Secretary John Kirby told reporters the Department of Defense did not have “perfect visibility” on the assault and could not provide exact numbers for how many troops Russian put ashore through landing ships.
The amphibious assault is a piece of the Russian plan to cut off the Donbas region and continue to target population centers, Kirby said.
The Russian navy largely played a supporting role in the Ukrainian invasion, which Russia began early Thursday morning, USNI News previously reported.
The amphibious assault comes from one of two amphibious task forces Moscow assembled for its attack on the neighboring country. Another task force was believed to be in Crimea and could land troops in the Black Sea port of Odesa.
On Sunday, Pentagon officials said Russian naval forces were beginning to assemble to the south of Odesa in the Black Sea but did not have indications of the intent of the forces.
“We do see some [naval] posturing in the Black Sea, south of Odesa. That is a little concerning, but it’s difficult right now to understand what that means,” the official said.
Moscow has a force of more than 10 amphibious ships, USNI News reported. The Kremlin has moved six amphibious landing ships into the Black Sea over the last month to join landing ships that were already part of the Black Sea fleet.
These include three Ropucha-class tank landing ships, which are capable of landing 10 main battle tanks and 350 troops ashore. The larger Ivan Gren-class, also in the Black Sea, can move 13 main battle tanks and 300 troops while also fielding two attack helicopters.
On Saturday, a senior defense official said the landing near Mariupol used a little less than half of the Russian tank landing ships in the Black Sea.
“Our assessment was that they used four of their landing ships to conduct this assault. We don’t know how much naval infantry was on each one, we just don’t have a perfect count,” the official said.
“But this would involve several thousand naval infantry troops.”
Meanwhile, two commercial vessels, off of the port in Odesa, were hit by Russian fire, according to Naval News. The Namura Queen, flying under the flag of Panama, took a missile to her stern.
Bunker Millenium Spirit, flying under the flag of Moldova, was also hit causing a fire, according to Naval News. The Ukrainian Ministry of Defence attributes the missile to Russia, while the Moldovan government said the source is unknown.
U.S. agribusiness giant Cargill confirmed that one of its chartered grain-carrying ships had been attacked in the Black Sea.
On Sunday, Pentagon officials confirmed the Russian Navy was targeting cargo ships in the Black Sea.
Russia also amassed 16 ships near Syria, USNI News reported.
Russian troops have been largely advancing along three lines, the senior defense official said, which remains true as of Sunday morning.
Forces are moving south to north to south, going from Crimea to the Ukrainian city of Kherson. Forces are now going both northwest and northeast from the south, the defense official said, which includes splitting some forces toward Mariupol.
Russia also moved troops from Belarus toward the Ukrainian capital Kyiv and toward Kharkiv, Ukraine. It is likely that Russia expected its troops to move into Kyiv quicker but has met unexpected resistance, the senior defense official said.
In the city of Chernihiv, about 90 miles outside of Kyiv, Pentagon officials say that Russian troops have taken up a siege posture as of Sunday morning and fired rockets into the city.
“They’re making a move on Kyiv from northwest of Kyiv, as well as from northeast of Kyiv,” a defense official said.
“On that northeast vector down towards Kyiv, they’ve had trouble around Chernihiv. And it appears that they are adopting a siege mentality, which any student of military tactics or strategy in history will tell you when you adopt siege tactics, it increases the likelihood of, of collateral damage to civilian infrastructure as well as to civilian life.”
The continued slowed movement might explain the Russian amphibious assault, as former NATO Supreme Allied Commander Adm. James Stavridis told USNI News on Thursday that amphibious landings become an option when the ground offensive is slowed.
“The Russian Black Sea fleet is essentially a holding force directed against the Ukrainian military south of Ukraine. Assuming Putin’s ground offensives don’t bog down – which will depend on the quality of Ukrainian resistance – they will probably limit their participation to surface-to-surface missile strikes,” Stavridis previously told USNI News.
Ukraine previously said that its troops would fight for the country. That was demonstrated in an incident on Snake Island, the senior official said.
A Russian warship opened fire on 13 border guards on Snake Island after telling them to surrender, according to The Washington Post. The border guards refused, telling the Russian warship to, “go fuck yourself.”
All 13 border guards were killed, according to Post reporting.
During the press conference, the senior official said the Snake Island incident demonstrated Ukrainian resistance.
“From our perspective, it’s both gut-wrenching and inspiring,” he said. “And certainly reflective of what we have seen in the last 24 hours, which is Ukrainians being willing to fight for their country, and do so bravely.”