U.S. paramedic Joseph Stone is the first casualty of an international monitoring mission in eastern Ukraine.
The OSCE monitor died in a landmine explosion in the region where Russian-backed rebels are fighting Ukrainian forces.
Organization for Security and Cooperation in Europe also fell victim to a cyber hack that used tools attributed to a Russian group.
International observers claim hostilities have increased by 40 percent in a week.
Outside observers have monitored U.S. elections since 2002.
Poroshenko says that he believes this will be most efficient guarantor of security.
The peace deal, which only came into force this weekend, already looks under threat.