Croatia's prime minister Saturday urged closer cooperation within NATO after a military drone, which he said had been launched from Ukraine, crashed in Zagreb.
The Soviet-era Tu-141 reconnaissance drone crashed in the Croatian capital late Thursday, damaging around 40 parked vehicles, but no one was injured.
It entered Croatia's airspace from Hungary, having flown in from Romania, officials said earlier.
All three countries are NATO members.
"According to what we know now it was obviously launched on Ukraine's territory", Prime Minister Andrej Plenkovic told reporters on Saturday.
"We don't know in whose possession it was," he said, adding that both Ukraine and Russia had said it was not theirs.
Plenkovic, speaking at the site of the incident, said he had sent a letter to his European Union counterparts and NATO Secretary General Jens Stoltenberg over the incident.
"Whether it was accidental, a mistake or intentional, we do not know at this moment."
The incident "points to the need for closer cooperation within NATO", he said.
The drone had flown undisturbed over the alliance's three member states, he said, adding: "We cannot tolerate such a situation any more.
"It was a very clear threat that requires a reaction."
According to officials, the drone flew over Hungary and Croatia before crashing.
Local security experts immediately labelled the incident a NATO failure. President Zoran Milanovic said Friday "It's a matter of NATO joint command".
On Friday, a NATO official told AFP that the military alliance's "integrated air and missile defence tracked the flight path of an object which subsequently crashed in Zagreb".
Later Saturday, Stoltenberg tweeted that he "spoke with Croatia's PM @AndrejPlenkovic on yesterday's drone incident in Zagreb.
"We agreed to stay in close contact and work together to establish the facts," he wrote.
The drone crashed around 11:00 pm (2200 GMT) Thursday in a park close to the Jarun lake.
Some six kilometres (four miles) from the city centre, the site is just next to a students residence with some 4,500 people and residential buildings.
The 14-meter (46-foot) drone, weighing more than six tonnes, still has to be dug out from a crater it made when crashing.
Zagreb is about 550 kilometres flying distance from the border with Ukraine, which Russia invaded on February 24.