European Union (EU) High Representative for Foreign Affairs Federica Mogherini raised concerns over the conflict in Ukraine on Tuesday, where rebels have claimed control of most of the strategic town of Debaltseve, despite a ceasefire having been agreed on Sunday.

"I would not say that the Minsk agreement has failed, but there is bad news. We knew it would be difficult, fragile, not black or white. However, we will not declare the failure of a process, but rather work for its success," Federica Mogherini said.

"The developments over the past hours are not encouraging and show that the agreements have to be reached. Minsk gives us fragile hope," she said.

"Ukraine needs an agreement to be applied totally and the Russians and separatists know very well that it is not only Europe but the international community which is looking to the implementation of these agreements, which are crucial for the country's future, region and civil population, which after almost a year of conflict is living a very dramatic situation," Mogherini added.

She said she had made contacts in the past hours with Germany after having talked over the telephone with German chancellor Angela Merkel and with Russian President Vladimir Putin and Ukrainian President Petro Poroshenko.

Mogherini also admitted on Tuesday that the EU has not given recent developments in Libya adequate importance.

"We did not realize the importance of this fact (the situation in Libya), but now our priority is to work with the UN. This is a threat for Europe," Mogherini stated in a press conference in Lisbon.

Her declarations to the media were made after meetings with President Anibal Cavaco Silva, at the Presidencial Palace in Belem, and Prime Minister Pedro Passos Coelho, Defence Minister Aguiar Branco and Minister of State and Foreign Affairs, Rui Machete.

"This situation is increasingly more worrying and the European union has a role to play to avoid Libya collapsing," she added.

The former Italian foreign minister was appointed as new EU high representative for foreign affairs last year and visited Portugal for the first time on this capacity on Tuesday.

Mogherini's remarks came amid a battle between rival militias for power four years after the revolution, when Muammar Gaddafi was forced to step down.

"The real solution for the challenges facing the southern borders (of Europe) is resolving the causes, starting from the situation in Libya, which in this area is the main problem," she pointed out.

Rui Machete also pointed to the proximity of Libya to Europe's southern borders and highlighted the Islamic State's "worrying influence" over sub-Saharan Africa.

He said Libya, where the Islamic State has controlled large swathes of territory, enabled the traffic of arms, humans and petrol, which fuelled radicalism and migration flows.

Other issues discussed between the EU official and Portuguese officials include peace in the Middle East.