The United States and the European Union have called on Belgrade to address their concerns over alleged “irregularities” in parliamentary polls won by the ruling Serbian Progressive Party (SNS).

“We conclude with concern that the electoral process [in Serbia] requires tangible improvement and further reform,” EU commissioners Josep Borrell and Oliver Varhelyi said on Dec. 19.

“The proper functioning of Serbia’s democratic institutions is at the core of Serbia’s EU accession process,” the two officials said in a joint statement.

Serbians began voting on Sunday in a snap election seen as an effort by President Aleksandar Vucic and his populist Serbian Progressive Party (SNS) to secure another four-year term, after two mass shootings earlier this year rattled their popularity.

A total of 18 parties and alliances are vying for the support of the 6.5 million-strong electorate for 250 seats in the parliament. The threshold for entering the parliament is 3% of votes. Polls opened at 7 a.m. (0600 GMT) and will close at 8 p.m.

The United States is monitoring a troubling Serbian military deployment along the border of Kosovo that is destabilizing the area, the White House said on Friday and called for the forces to be withdrawn.

White House national security spokesperson John Kirby told reporters that Kosovo's peacekeeping force is going to be increasing its presence of NATO forces in northern Kosovo as a result of the tensions.

Serbia has agreed to supply arms to Kyiv or has sent them already, according to a classified Pentagon document, despite the country's professed neutrality in the Ukraine war and refusal to sanction Russia over its 2022 invasion.

The document, a summary of European governments' responses to Ukraine's requests for military training and "lethal aid" or weapons, was among dozens of classified documents posted online in recent weeks in what could be the most serious leak of U.S. secrets in years.

Kosovo Prime Minister Albin Kurti and Serbian President Aleksandar Vucic arrived in North Macedonia on Saturday for a fresh round of talks with EU officials on implementing a deal to normalise ties between Belgrade and Pristina.

The two leaders will hold separate meetings with European Union foreign policy chief Josep Borrell before a three-way session and a news conference expected later in the day. 

"I am optimistic," Kurti said ahead of the meetings, taking place in the lakeside town of Ohrid in North Macedonia.