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.
Interior ministry battalions are also at the forefront of the 15-month battle against separatist fighters that has claimed more than 6,600 lives.
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.
Pravy Sektor reported the death of two of its members in a shootout with what appeared to be a mixture of security personnel and armed local thugs.
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.
- 'Order through force' -
Pravy Sektor leader Dmytro Yarosh -- a bombastic parliament member who was wounded in eastern Ukrainian clashes and whom Moscow identifies as a neo-Nazi -- pleaded for calm.
"We are working with the Ukrainian Security Service in order to stabilise the situation," he wrote Monday on Facebook.
The gun battle erupted in a rural region split from the rest of Ukraine by the Carpathian Mountains and has ancient cultural ties to Hungary.
The Zakarpattia (Trans-Carpathian) province has often wound up siding with pro-Russian political parties because many locals also mistrust the Ukrainian nationalists.
About half of the region's residents also carry Hungarian passports and have few political links to either Moscow or Kiev.
The Ukrainian government's near silence on the crisis was broken Monday when Prime Minister Arseniy Yatsenyuk came out firmly on Pravy Sektor's side.
Yatsenyuk has issued orders "to dismiss all Zakarpattia customs service employees," his office said in a statement.
The politically more cautious Poroshenko appeared to prefer the middle ground by blaming both sides.
The president opened Monday's security meeting by accusing "the Zakarpattia smugglers of throwing down a challenge to all of Ukraine."
But he also blamed Pravy Sektor -- whom he called "true patriots" -- of allowing "banal criminal gangs" enter its ranks.
A senior Ukrainian security service source said most in the government believed Pravy Sektor was far less innocent than it claimed.
Pravy Sektor "itself is trying to win a piece of the contraband business," the security source said.
"The problem for Ukraine is that there are a lot of armed people who view themselves as heroes and try to establish their brand of order through force," said the source.
"Something like this was bound to happen -- and it did."