Ukraine on Friday charged a prisoner swap negotiator with plotting to assassinate President Petro Poroshenko and other top officials leading the four-year campaign against Russian-backed eastern insurgents.
A judge in Kiev said Volodymyr Ruban planned to use mortar fire and grenades to attack the presidential administration and parliament with the help of the rebel command.
"I would like to report that the terror threat level facing Ukrainians was, in our view, the highest in the history of our independence," Ukrainian Security Service chief Vasyl Grytsak told Poroshenko in a televised meeting.
Ruban was detained on Thursday with a haul of heavy ammunition in his vehicle while crossing the "grey zone" splitting the war zone from the rest of Ukraine.
The 50-year-old Ukrainian national is known to have organised several prisoner exchanges between the two sides since clashes, which have claimed more than 10,000 lives, began in 2014.
Ukrainian media had previously accused Ruban of harbouring pro-Russian sentiments and being supportive of the separatist cause.
"Ruban is suspected of preparing armed attacks against senior government officials," the presiding judge in Kiev was quoted as saying by Interfax-Ukraine news agency.
The judge said Ruban intended to use "mortar guns, grenade launchers, light weapons and explosive devices" to assault the government quarters in Kiev and officials' homes.
Ukraine's deputy head of the war-tattered east later posted a photo on Facebook that he said showed the weapons cache seized from Ruban's minibus by the military.
A dozen Kalashnikov rifles and an assortment of heavy munitions were laid out on top of storage boxes that security officials said partially belonged to the Russian army.
Ruban told the court he did not know that the bus he was using to cross into Kiev-controlled territory was carrying the weapons.
He was placed in pre-trial detention for two months.
The alleged targets are all hate figures in Moscow who have overseen the Ukrainian military campaign and the February 2014 pro-EU revolt that pulled Kiev out of Moscow's orbit.
These included Interior Minister Arsen Avakov and National Security and Defence Council chief Oleksandr Turchynov -- viewed by Russia as the war's main proponents.
The judge said Ruban wanted to "create chaos... and help Russia launch a full-scale offensive" that would put the separatist region under its permanent control.
Russia rejects either plotting or backing the fighting in order to destabilise its western former Soviet neighbour as it slowly charts closer ties with the West.
There was no immediate response to Friday's news from Moscow or the separatist leadership in the east.
But the case threatens to further undercut any attempts to put a negotiated end to Europe's only military conflict.
Grytsak told reporters that his investigation would provide evidence implicating the separatist leadership of the self-proclaimed Donetsk People's Republic (DNR).
"We have obtained sufficient evidence to show that Ruban acted in collusion with DNR leader (Aleksandr) Zakharchenko," the security service head said.