An EU official and an EU diplomat stated that ambassadors from EU member states have been invited to a meeting of the bloc’s crisis response working group on Monday to discuss worries about an escalation of the violence in Ukraine.

The talks will also include the continuing referendums in seized parts of Ukraine aimed at annexing land to Russia, as well as how the EU intends to manage dozens of Russian men attempting to enter the EU to avoid a partial mobilization imposed last week, according to the sources.

Ambassadors are not usually present at meetings of the integrated political crisis response group, which includes officials from the EU’s executive and its diplomatic service.

The closed-door meeting in Brussels was due to start at 10 a.m. (0800 GMT), the sources said.

The referendums in four eastern Ukrainian regions, which Kyiv and the West regard as a sham, entered a fourth day on Monday, after the United States warned of “catastrophic consequences” if Moscow used nuclear weapons to protect any annexed regions.

The warning followed Wednesday’s thinly veiled nuclear threat by Russian President Vladimir Putin, who said Moscow would use any weapons to defend its territory.

Russian forces control territory in the four eastern Ukrainian regions, which represent about 15% of Ukraine and are roughly the size of Portugal.

It would add to Crimea, an area nearly the size of Belgium, which Russia claims to have annexed in 2014.

Russia’s parliament could move to formalise the annexations within days.

By incorporating the areas of Luhansk, Donetsk, Kherson and Zaporizhzhia into Russia, Moscow could portray efforts to retake them as attacks on Russia itself.