EU leaders will meet to decide their next move after Russia annexed Crimea earlier this week, which one top European diplomat on Wednesday called "the most flagrant breach of international law since the Second World War."
Heads of state and government will also start discussing potential economic sanctions against Moscow, although no immediate decision is expected.
It comes after EU foreign ministers decided on Monday to slap sanctions on 21 political and military figures.
Officials in Brussels say that more names could be added to that list during an EU summit on Thursday and Friday.
Vladimir Putin signed a treaty allowing Crimea to split from Ukraine and join Russia on Tuesday; the West says the move is illegal and illegitimate.
Tougher economic sanctions would come at a price to Europe's economies.
Many are heavily reliant on Russian energy, whilst export-driven EU members such as Germany sell billions of euros-worth of goods to Russia each year.
Euronews met with Lithuanian Foreign Minister Linas Linkevičius on Monday to ask him what he expected from the summit and whether the EU should take a tougher line on Russia.
James Franey, euronews: Can you explain how Lithuania's history shaped your view of this Ukrainian crisis?
Linas Linkevičius, Lithuanian Foreign Minister: I think it's a personal legacy you know. It's always important something that took place not far back ago. It's always quite fresh. Sometime feelings of uncertainties; sometimes feeling that you should cope with the problems yourself. But now its a bit different. We are members of the European Union; we are members of NATO. So we can plan collectively so how to withstand the threats, the pressure.
euronews: So you don't fear any Russian interference inside your country?
Linkevičius: As I said, being members of NATO, we have security guarantees. But our neighbours do not have these guarantees. And the least that we can do to show our solidarity and send a very clear messages. What shouldn't be done in the 21st century: invasion, aggression against [a] sovereign country, which is unfortunately happening not for the first time. And in order to avoid this happening again, we should do our best, our utmost...at least what we can do, collectively to prevent this.
euronews: What does the best look like? What do you expect to see on Thursday and Friday?
Linkevičius: Frankly speaking, the language of sanctions is nothing that we should be proud about, because it's really the last resort. When calls, invitations, requests to calm down, to engage Russia directly with the Ukrainian government, also to deescalate the situation. All these calls, expressions and statements of concern are important but the time comes probably when some more tangible probably signals should be sent.
euronews: So if I could put it another way: what does Russia need to do to avoid these tougher economic sanctions? This third step that we are talking about..concretely.
Linkevičius: The occupation or annexation of Crimea is No Go. We will never recognise that. It's as clear as it is. But now it's not less important to talk about further escalation in the eastern and southern part of Ukraine. Because no-one can guarantee that it will not happen, although it was said that it was not on the table. Nobody is discussing this issue, but we see that the same scenario, as I said, (self)-inflicted conflicts. Russian, so to say, citizens and the need to defend them. To think about the possibility of intervening by force is really alarming and is really concerning us. And we should really react to prevent that happening. So (it's) not less important than to talk about the de-facto annexation of Crimea, which is already assessed as a violation of international law.
euronews: Let me just push you on where this red line is.. I'm trying to figure out what Russia needs to do exactly.
Linkevičius: I think personally that that line has already been crossed. Yeah, it's already done. What else should be done in order to convince all that what is going on is illegal. If you are asking, if somebody needs more...that more is really a further escalation of the situation
euronews: A move into Eastern Ukraine perhaps?
Linkevičius: A move so to say into other parts of Ukraine. That could be even more so to say a crossing of this line so maybe that would be more convincing enough to be more consistent in our way. The least we could do would be to implement what was already decided. So that would also be a clear signal.
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