The EU has threatened to impose more sanctions on Russia following its new offensive in south-east Ukraine.

EU foreign relations chief Federica Mogherini on Saturday (24 January) said “further escalation … would inevitably lead to a further grave deterioration of relations”.

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Her communique spoke of “offensives by Russia-backed separatists”, adding, “I call therefore openly upon Russia to use its considerable influence over separatist leaders and to stop any form of military, political, or financial support”.

The same day EU Council president Donald Tusk tweeted: “Once again, appeasement encourages the aggressor to greater acts of violence. Time to step up our policy based on cold facts, not illusions".

Foreign ministers are to discuss the situation at an emergency meeting in Brussels on Thursday.

Latvia's Edgars Rinkevics said on Twitter at the weekend: “Russia fully responsible to stop them [the attacks], if not, more isolation & sanctions to come”.

His Lithuanian and Swedish counterparts issued similar warnings.

But German foreign minister Frank-Walter Steinmeier said: "Efforts for de-escalation need to be continued. I hope that - even after the past three days - not everything is lost".

They spoke after Russia-controlled fighters seized the airport in Donetsk, east Ukraine, last week and, on Saturday, fired rockets into a market in Mariupol, in the south-east of the country, killing dozens of civilians.

Nato secretary general Jens Stoltenberg said after the Mariupol attack: “Russian troops in eastern Ukraine are supporting these offensive operations with command and control systems, air defence systems with advanced surface-to-air missiles, unmanned aerial systems, advanced multiple rocket launcher systems, and electronic warfare systems”.

For its part, the UK also on Saturday proposed a UN Security Council (UNSC) resolution calling for violence to stop.

But Russia, a UNSC veto member, blocked it on grounds it didn't criticise the Ukrainian government.

One EU diplomat based in Kiev told EUobserver the snap FMs meeting is unlikely to agree on sanctions right away.

“The feeling among EU countries’ diplomats here on the ground, even from the more Russia-friendly states, is that there should be some kind of reaction. But the feeling in the capitals is different”, the contact said.

“Foreign ministers could meet but only to issue a strong statement".

The EU last year blacklisted dozens of Russian officials, oligarchs, and MPs. It also imposed sanctions on Russian banks, energy, and arms firms, and a ban on EU investments in Russia-annexed Crimea.

Its option papers say that in the worst case scenario it could stop Russian gas and oil purchases.

Some member states, including the UK, have also suggested excluding Russia from the Belgium-based “Swift” system of international wire transfers.

But for his part, Sergei Markov, the director of the Moscow-based Institute of Political Studies, who describes himself as an advisor to Russian president Vladimir Putin, indicated the Russian elite doesn’t believe the EU threats.

He wrote on his Facebook page on Saturday that if the “New Army” takes Mariupol, the EU and US might impose some extra measures.

But he said if it also takes Kharkiv and Odessa, two cities in Ukraine, the EU would launch new talks with Russia and “gradually lift all sanctions … in exchange for not taking Kiev”.

The EU diplomat noted: "This is a message to the EU ... they think we'll do anything to avoid a full-scale war".

One EU, two voices

Meanwhile, the Mogherini and Tusk statements received a mixed reception.

Mogherini's language is tougher than previously on Russia. But Ukrainian activists attacked her on social media for speaking of “Russia-backed separatists” and Russian “influence” in what appears to back Russia's claim that it isn't directly involved in the conflict.

Jacek Saryusz-Wolski, a Polish centre-right MEP, also tweeted: “Russia as aggressor should ‘use its considerable influence’ on itself, not its terrorists, [please] refrain [from] repeating RU [Russian] propaganda”.

The Tusk tweet, which pointed to Russia as “the aggressor”, was welcomed in Ukraine and by Russia hawks in Europe.

“Tusk nails it”, Estonian president Toomas Hendrik Ilves said.

This story was updated after Mogherini on Sunday called next week's foreign ministers meeting