Borrell stressed however that the troop increase in Bosnia is a "precautionary measure" in the wake of the February 24 invasion and secessionist threats made by a Bosnian Serb leader allied with Moscow.

"We have reinforced our ability to react swiftly in case of crisis", Borrell told the European Union force in Bosnia (EUFOR) outside Sarajevo.

"And we will continue deterring those who would feel emboldened to undertake destabilisation actions."

While referring to the "brutal Russian attack on Ukraine", the EU's top diplomat underlined that the EUFOR presence is "more important then ever".

"In this tense moment, I want to reassure the people of Bosnia and Herzegovina of our unwavering commitment to maintain safety and stability", Borrell told the soldiers.

On the same day that the Russian offensive in Ukraine began -- February 24 -- EUFOR announced the deployment of 500 troops to Bosnia to reinforce the 600 already in place.

Borrell stressed that the move should be understood only as a "precautionary measure," however.

"As we speak, right now, there are not serious threats to the safety and security on the environment in Bosnia and Herzegovina", the Spanish diplomat told reporters later at the base.

Ever since the brutal 1990s war that claimed some 100,000 lives, Bosnia has been divided on ethnic lines. One half of the country belongs to the Serb entity while the other is ruled by a Muslim-Croat federation.

However last December, after years of fragile peace, Serb political leader Milorad Dodik launched a process of Serb withdrawal from the army, judiciary and the tax system, stirring fears of breaking up the country or starting a new conflict.

Before making the move, Dodik travelled to Moscow, appearing to have tacit backing from Russia, the Serbs' historic ally.

Dodik -- who was slapped with US sanctions in January -- recently said he has no plans of secession.