Hungary's foreign minister said Wednesday the government has ordered the closure of the EU country's border with Serbia and the construction of a fence along its length to keep out migrants.

"The Hungarian government has instructed the interior ministry to physically close the border with Serbia," Peter Szijjarto told reporters.

EU member Hungary threatened Friday to close its border with Serbia to stop an influx of migrants.

"We don't think it's right that they send us the refugees, they need to be stopped on Serbian territory," Hungary's Prime Minister Viktor Orban said in an interview on Kossuth public radio.

New anti-immigration billboards put up by the Hungarian government have sparked political controversy, with the head of a small opposition party admitting on Monday that he had vandalized the state-funded posters. 

The posters, which recently popped up in the capital Budapest, feature slogans like: "If you come to Hungary, you cannot take away Hungarians' jobs" and "If you come to Hungary, you have to respect our culture!" 

Hungary's prime minister said on Friday he wanted to improve the European Union but not leave the bloc, as his British counterpart David Cameron courted European capitals for EU reform.

"I am ready to criticize the EU on any issue, and I am ready to debate with European bureaucrats, but I am firmly opposed to, and we should fight, anybody who would steer the country in a direction of exiting the EU or NATO," Viktor Orban said in a speech marking his five years in government.

The United Nations voiced outrage Friday about a questionnaire distributed last week in Hungary suggesting a link between immigration and terrorism, warning it could boost xenophobia in the country.

"We are shocked by the prime minister's introductory message, which suggests a link between migration and terrorist attacks, including the attack which took place in Paris in early January this year," Cecile Pouilly, spokeswoman for the U.N. human rights agency, told reporters.

A suspected homemade bomb was found this weekend on a bus in Hungary that had 34 passengers on board, prompting a probe by the EU's policing agency, the driver and officials said Monday.

"A flammable substance was found on the afternoon of May 16 on a bus registered in Bulgaria and traveling in Hungarian territory," Bulgaria's foreign ministry said in a statement.

The leader of Iraq's autonomous Kurdish region, Massud Barzani, said on Monday his forces had "broken the spine" of the Islamic State (IS) group with the help of international allies.

"Thanks to (the Kurds') opposition and the help of their friends in the coalition, the spine of the Islamic State is broken," Barzani said after talks with Hungarian Prime Minister Viktor Orban in Budapest.

Hungarian Prime Minster Viktor Orban raised again on Friday the thorny topic of the death penalty in the European Union, saying member states should be able to decide on the issue themselves.

"We want to form European public opinion toward bringing back to national competency level the question of whether to introduce the death penalty or not," Orban said in a radio interview.

Hungarian Prime Minister Viktor Orban "does not plan to introduce the death penalty," his chief of staff Janos Lazar said Thursday, after strong EU criticism of Orban's call for debate on its reintroduction.

Orban informed European Parliament president Martin Schulz by telephone that the government would debate the issue, but "the prime minister does not plan to introduce it in the country," Lazar said.

Hungary will send up to 150 soldiers to Iraq to support the U.S.-led coalition's fight against the Islamic State group, lawmakers decided on Tuesday.

The troops' main role will be to protect a training center in Iraqi Kurdistan's capital, Arbil.