Flowers, candles and a picture of Kosovo Serb politician Oliver Ivanovic, who was shot dead Tuesday morning by still-unknown assailants, at the scene of the shooting in front of his office in the northern, Serb-dominated part of Mitrovica, Kosovo, Tuesday, Jan. 16, 2018. People from Kosovo's Serb minority say they are in shock over the killing of a moderate politician who was gunned down in an attack in a northern town. (AP Photo/Bojan Slavkovic)

Flowers, candles and a picture of Kosovo Serb politician Oliver Ivanovic, who was shot dead Tuesday morning by still-unknown assailants, at the scene of the shooting in front of his office in the northern, Serb-dominated part of Mitrovica, Kosovo, Tuesday, Jan. 16, 2018. People from Kosovo's Serb minority say they are in shock over the killing of a moderate politician who was gunned down in an attack in a northern town. (AP Photo/Bojan Slavkovic)

MITROVICA, Kosovo (AP) — A leading Serb politician was shot to death Tuesday near his political party’s offices in northern Kosovo, an attack that raised ethnic tensions in the Balkans and prompted the suspension of EU-mediated talks between Kosovo and Serbia.

Unknown assailants opened fire on Oliver Ivanovic, 64, in the Serb-controlled northern city of Mitrovica. Ivanovic was taken to a hospital, but doctors were unable to save him.

An autopsy showed he was shot six times in the upper torso. The assailants escaped in a car that was later found burned out. Kosovo police sealed off the area of the shooting while they searched for suspects.

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Ivanovic was one of the key politicians in Serb-dominated northern Kosovo, a former Serbian province where tensions remain high a decade after Kosovo declared independence in 2008. Serbia does not recognize Kosovo as a separate country.

Ivanovic was considered a moderate who maintained relations with NATO and EU officials after Serbia lost control of northern Kosovo following NATO’s 1999 bombing to stop a deadly Serb crackdown on ethnic Albanian separatists.

Ivanovic, who was married with three children, had enemies both among Kosovo Albanians and nationalist Serbs because of his moderate policies.

A Kosovo court convicted him of war crimes from the 1998-99 Kosovo war. The verdict eventually was overturned and a retrial was underway.

In Pristina, the Kosovo government strongly denounced the slaying, saying it considered the attack a challenge to “efforts to establish the rule of law in the whole of Kosovo territory.” Kosovo police offered a 10,000-euro ($12,250) reward for information about the attackers.

In Belgrade, Serbian President Aleksandar Vucic held a top security meeting to discuss the attack. Vucic called Ivanovic’s killing “a terrorist act” and said Serbia would demand to be included in any investigations carried out by international missions based in Kosovo.

“Serbia will take all necessary steps so the killer or killers are found,” he said.

Serbian Foreign Minister Ivica Dacic said the killing threatened the stability of both Serb-populated northern Kosovo and the whole Balkan region.

After a meeting of Kosovo’s National Security Council, Kosovo Prime Minister Ramush Haradinaj rejected Serbia’s demand to take part in the investigation and said he might invite the FBI.

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Haradinaj also said that Ivanovic’s slaying was the result of “illegal involvement in the north of other institutions beyond Kosovo.” He didn’t elaborate on what outside forces he was accusing of being involved in Ivanovic’s slaying.

At the news of Ivanovic’s death, the Serb delegation at a previously scheduled session of the EU-mediated talks immediately left Brussels to return to Belgrade.

“Whoever is behind this attack ... whether they are Serb, Albanian or any other criminals, they must be punished,” delegation leader Marko Djuric said.

Avni Arifi, who heads the Kosovo delegation to the talks, called on Serbia to return to the negotiations.

“There is no alternative to the dialogue,” Arifi told Klan Kosova TV.

European Union foreign policy chief Federica Mogherini called the presidents of Serbia and Kosovo to express the EU’s condemnation of the killing. She appealed for both sides “to show calm and restraint.”

The head of the OSCE Mission in Kosovo, Ambassador Jan Braathu, said he was “shocked and deeply saddened” and considered Ivanovic “among the most prominent Kosovo Serb representatives for almost two decades.”

He also urged “all sides to avoid dangerous rhetoric and remain calm at this sensitive time, and recommit themselves to continue the work toward the normalization of relations and improvement of the lives of the citizens of Kosovo and Serbia.”

NATO also urged Serbia and Kosovo to return to the talks, which are aimed at normalizing relations their relations.

“NATO fully supports the EU-facilitated dialogue between Belgrade and Pristina, and calls for this dialogue to continue as soon as possible. This is critical for regional peace and security,” NATO spokeswoman Oana Lungescu said.

Lungescu urged “all parties to exercise restraint to defuse tensions, and allow the judicial authorities to carry out a full investigation.”

A NATO-led peacekeeping force established in 1999 “continues to guarantee a safe and secure environment and freedom of movement throughout Kosovo” following Tuesday’s slaying, Lungescu said.

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This version corrects the spelling of NATO spokeswoman Oana Lungescu’s last name.

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Dusan Stojanovic and Jovana Gec in Belgrade; Llazar Semini in Tirana, Albania; and Raf Casert in Brussels contributed to this report.