The EU-backed leader convened his "military cabinet" of top generals to try and diffuse tensions between two forces vital to his bid to stamp out a separatist insurgency 1,000 kilometres (more than 600 miles) away in eastern Ukraine.
The Pravy Sektor (Right Sector) nationalists played a small but instrumental role in three months of pro-European protests that eventually toppled the Kremlin-backed leadership in February 2014.
They then formed the heart of some of the best-equipped -- and allegedly most ruthless -- units to wage war against pro-Russian militias that overran parts of Ukraine's eastern industrial heartland in the wake of the Kiev revolution.
But mistrust between volunteer units and Ukraine's armed forces has been building as Poroshenko tries to follow through on a February peace deal whose terms -- including partial self-rule for the insurgents -- are anathema to the nationalists.
Pravy Sektor members also cast themselves as Robin Hood figures who defend the public against corruption that has ravaged Ukraine's political establishment and police since the Soviet Union's collapse in 1991.
The weekend battles were sparked by Pravy Sektor's self-proclaimed bid to prevent Hungary and Slovakia from being flooded with any more contraband cigarettes that have already enriched local politicians and their security overlords.
The interior ministry -- backed up with armoured personnel carriers and military trucks -- said one civilian had also died in circumstances that were unclear.
The battle broke out after a dozen or so Pravy Sektor members came to what was meant to be a round of negotiations equipped with grenade launchers and machine guns.
Both sides accuse the other of being the first to open fire. The Pravy Sektor groups claims to have slipped through a security cordon and established new positions in the surrounding woods.