Members of the police searching the fields near the village of Przewodow, a day after two people were killed in an explosion at a farm by the Polish village.
CNN  — 

Poland’s ambassador to the United Nations says the country acted with “full restraint” after a suspected Russian-made missile fell inside its border on Tuesday, killing two Polish citizens and sparking fears of an escalation in Russia’s war against Ukraine.

Speaking at the UN Security Council (UNSC) on Wednesday, Krzysztof Szczerski said Poland immediately launched an “extensive multiphase investigation” into the incident, which Warsaw and NATO said was likely caused by a missile fired by Ukrainian forces defending their country against Russian strikes.

“Poland is also conducting intensive consultations on the incident with its NATO allies and key partners,” he said, noting that the incident prompted Poland to increase the combat readiness of a unit of the armed forces focused on airspace monitoring.

Two farmers died when the missile caused an explosion outside the rural eastern Polish village of Przewodow, about 4 miles (6.4 kilometers) west of the Ukrainian border.

Tuesday’s incident marks the first time a NATO country has been directly hit during the nearly nine-month conflict and prompted an emergency meeting of ambassadors from the US-led military alliance in Brussels Wednesday.

Szczerski said the incident “teaches us how close we actually live the potential escalation in the spillover of Russia’s war of aggression against Ukraine of far-reaching consequences that we all can perceive.”

He added that the two Polish victims “would not have been killed if there had been no Russian war against Ukraine.”

“Their only fault was the fact that they lived close to the civilian infrastructure on the Ukrainian side of the border that Russia keeps attacking as a military target,” he said.

NATO head Jens Stoltenberg said Russia bears “ultimate responsibility” for the incident, “as it continues its illegal war against Ukraine.”

The blast occurred roughly the same time as Russia launched its biggest wave of missile attacks on Ukrainian cities in more than a month, predominantly targeting energy infrastructure.

The Polish ambassador repeated his country’s early assessment that the incident appears to have been unintentional.

“The very initial findings support the hypothesis that the event was not a deliberate attack. But naturally, we need to wait for the final conclusion until the investigation is over,” Szczerski told the UNSC.

Investigations at the site where the missile landed will continue to be a joint operation with the United States, Polish President Andrzej Duda said Wednesday. Ukrainian President Volodymyr Zelensky has called for Ukrainian experts to be allowed to the site.

The US National Security Council said it has “full confidence” in the Polish investigation into the blast and that the “party ultimately responsible” for the incident is Russia for its ongoing invasion.

CNN’s Phil Mattingly, Kevin Liptak, Radina Gigova, Jim Sciutto and Sophie Tanno contributed reporting.