Bulgaria's centre-right GERB party is projected to win Sunday's snap parliamentary election, according to a pair of exit polls.

GERB, led by ex-Prime Minister Boyjo Borisov, won between 32.2 percent and 32.8 percent of the vote, while the Socialists trailed behind with 28 percent to 28.4 percent, exit polls by Alpha Research and Gallup International Balkan (GIB) said.

Borisov said the election result "confirmed categorically that GERB should be the leading ruling party", adding that he would make "utmost efforts" with potential coalition partners to form a government quickly.

Socialist party leader, Kornelia Ninova, conceded defeat late on Sunday, but said the party would look at options for forming a government should the GERB party fail to do so.

"If they fail to form a government and we receive a mandate, we will try to form a Bulgarian government" to ensure stability in the country, Ninova told reporters.

Three other parties were projected to win at least four percent of the votes and qualify for the parliament: the rightist United Patriots coalition, with 8.8 percent; the Party for Rights and Freedoms of ethnic Turks with 7.7 percent; and newcomers Volya with 4.6 percent.

Official results are expected on Monday. If they confirm the exit polls, Borisov will be handed a mandate to form his third cabinet.

Forming a government after Bulgaria's third election in four years will be tough, however, and the resulting coalition may be short-lived, experts say.

Reporting from the country's capital, Sofia, Al Jazeera's Matea Damjanovic said the latest exit polls suggest six parties will enter into the parliament.

She added that "Borisov and his political party, GERB, will have to form a coalition, and probably they will do it with the United Patriots and Volya".

Bounce-back Borisov

A former firefighter and bodyguard, 57-year-old Borisov has been a towering figure in Bulgarian politics in recent years.

He has been premier twice but resigned both times - once in 2013 after nationwide protests against poverty and most recently in November after the loss of GERB's presidential candidate to Rumen Radev, a former air force general backed by the Socialists.

Borisov said he wanted a third mandate "to guarantee stability".

"I voted for a stable, predictable and united Bulgaria, because tomorrow our nation needs to be united," Borisov said after casting his ballot.

Along with the immigration, the election campaign focused on the future of the European Union, which Bulgaria joined in 2007, and the influence of Russia and Turkey on domestic politics.

The Socialist Party of ex-communists has pledged to improve economic relations with Russia, appealing to voters who feel let down by the EU. Bulgaria is also a member of NATO.

Socialist leader Ninova wants EU sanctions against Russia lifted, seeks a bigger role for the state in the economy and has wooed voters with promises of higher salaries and pensions.

Ninova said she voted on Sunday "for security at our borders and inside the country, for justice, and lastly not to give an opportunity to another country, no matter if it comes from East, West or South, to interfere in our politics".

A caretaker cabinet took over in late January, shortly after Radev was inaugurated and in position to dissolve the parliament and schedule the elections.

But analysts have warned that the resulting coalition may not last long.

"It seems that some configuration of political parties who support the oligarchic government model will win the elections," political analyst Evgeni Daynov told the AFP news agency.

"But because society has already realised how dangerous corruption is, this will inevitably lead to a highly unstable government."