He said: “Where our focus will be over the course of the coming days is on evasion.
“I think we’ll have some announcements in the next week or two that identify targets that are trying to facilitate that evasion both inside Russia and beyond.
“The president is actively looking at how we can deal with the fact that as we seize these assets, our goal is not to give them back.
“Our goal is to put them to a better use than that. But I’ll be careful in what I say today because there’s an ongoing – kind of – policy process around how we end up dealing with that question.
“But rest assured that the goal is not just to sit on them for a while and then pass them all back.”
Mr Sullivan went on to say that attempts by Moscow to disrupt American weapons being sent to Ukraine would escalate into a full blown conflict.
He said: “The United States is not operating inside the territory of Ukraine, so if the Russians, obviously, were to strike NATO territory, where material is being assembled, that would invoke Article 5 and would be a complete game changer.”
Article 5 of the NATO charter invokes the idea of collective self defence and says an attack on one member of the military alliance is an attack on all of its members.
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It has only once ever been invoked after the September 11, 2001, attacks on the United States.
Sullivan also added that the attack on the Russian missile cruiser Moskva, claimed to be fronted by Ukraine, had dealt a blow to Russia.
He said: “We’ve been in touch with the Ukrainians overnight, who had said that they struck the ship with anti-ship missiles.
“We don’t have the capacity at this point to independently verify that but certainly, the way this unfolded, it’s a big blow to Russia.”
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In contrast, Russia’s Defence Ministry said a fire had broken out on the missile cruiser Moskva, the flagship of its Black Sea fleet, after ammunition blew up on board, and did not mention an attack.
The ship has subsequently sunk.