European Union leaders gather in Brussels Saturday to thrash out a response to Russia's deeper involvement in war-torn Ukraine and to bring new faces to key EU posts.

The 28 heads of state and government sit down for talks as accusations rain down on the Kremlin that Russian combat troops are fighting inside Ukraine, with fears growing that Europe may soon face a full-scale war on its eastern borders.

It has only been five weeks since EU states toughened their sanctions against Russia, but with the West's almost unanimous belief that Moscow is now a direct and hostile force inside Ukraine, the EU is under pressure to once again up the stakes.

"Such behavior cannot remain without consequences," German Chancellor Angela Merkel said Thursday amid the reports by NATO that "well over" a thousand Russian soldiers were waging war in Ukraine.

British Prime Minister David Cameron urged Russia to take a different path and if it did not, "then she should be in no doubt that there will be further consequences."

Ukraine's embattled government has openly asked the EU to help militarily, and the plea, which has virtually no chance of being satisfied, will be relayed to the summit by Romanian President Traian Basescu.

Ukrainian President Petro Poroshenko is a last-minute guest at Saturday's talks, invited to share the latest developments after one-on-one meetings with Commission head Jose Manuel Barroso and EU Council President Herman Van Rompuy.

Poroshenko has been a regular presence amongst EU leaders since taking office earlier this year. In June he attended an EU summit to sign an Association Agreement, the very deal that sparked the crisis that the UN said Friday had killed 2,600 people.

Also during the talks, leaders will seek to agree on a successor to Catherine Ashton, who steps down in November after five years as the EU's foreign affairs supremo.

Leaders will also look to name a new EU President to replace, the quiet yet efficient Belgian, Van Rompuy.

Filling the EU's top jobs can be a painstaking process, with national pride, regional dynamics, and more recently an ambition for gender parity, also coming into play.

But with the crisis boiling over in Ukraine, hopes are that the European leaders may swiftly move on from haggling over jobs to tackling the more pressing issues.

French President Francois Hollande meanwhile is in a push to add economic policy to the agenda, in an effort to evoke alternatives to the balanced budget mantra that rules EU policy, driven by powerhouse Germany.

Favorite to replace the British Ashton is Federica Mogherini, Italy's 41-year-old foreign minister, hailed by her supporters as a new, younger face for Europe.

Mogherini's candidacy initially faced fierce resistance, with eastern European countries criticising her as both inexperienced and too soft on Russia and she was successfully sidelined at a first top jobs summit in July.

But six weeks later and after Italy staunchly backed more sanctions against Russia in July, Mogherini seems well placed.

This would leave the key EU president job as the real unknown for Saturday's deliberations.

For now, Donald Tusk, the current Prime Minister of Poland seems to have the edge.

Backed by Angela Merkel, Germany's chancellor and the EU's most powerful leader, the Polish premier, who speaks neither English or French, seems to have overcome a personal clash with Britain's Cameron over migration policy to be favorite.

The EU president job is primarily one of coordination, requiring a deft touch to navigate the bloc's leaders in laying the groundwork for summits.

A longer-shot is Denmark's Prime Minister Helle Thorning-Schmidt, probably best known for her selfie with Obama. A moderate social-democrat, very well-liked by the British and the media, she could be bring a much-needed sprinkle of stardust to the European institutions.

The EU is effectively led by a trio and filling out the group is the already named Jean-Claude Juncker, a grizzled European political veteran whose nomination to head the European Commission by EU leaders in June was met with fierce resistance from Cameron.

The former Luxemburg premier will replace Barroso and is in the process of filling posts for the next Commission with nominees provided by each member state.

Juncker's main problem is the lack of women. Barroso's outgoing Commission has nine women and so far governments have sent Juncker only four, including Mogherini.

Lawmakers in European parliament, who must ratify his choices, have warned they would oppose a college of European commissioners with fewer women than today. (AFP)