Arrests over alleged corruption at the European Parliament are “very, very worrisome”, the EU’s foreign policy chief Josep Borrell has said.
And Irish Foreign Affairs minister Simon Coveney said the charges against four people were “very damaging”.
Among the arrested was European Parliament Vice President Eva Kaili – believed to be one of those charged.
Prosecutors suspect a Gulf state – reportedly Qatar – tried to influence parliament by donating money or gifts.
Qatar has denied misconduct.
Belgian police seized cash worth about €600,000 ($632,000; £515,000) in 16 searches in Brussels on Friday. Computers and mobile phones were also taken, to examine their contents.
Four people have been charged while two have been released, prosecutors said on Sunday.
No suspects have been publicly named but Ms Kaili is understood to be among those who have been indicted.
“They are charged with participation in a criminal organisation, money laundering and corruption,” the Belgian federal prosecutor’s office said in a statement.
Ms Kaili – an MEP for eight years – was suspended from her duties as one of 14 vice-presidents by president of the European Parliament Roberta Metsola.
She has also been suspended from the parliament’s Socialists and Democrats Group, and expelled from the Greek centre-left Pasok party.
Prosecutors in Greece have reportedly frozen all of Ms Kaili’s assets.
Watchdogs and opposition MEPs said the bribery investigation could represent one of the biggest corruption scandals in European Parliament history.
“Certainly the news is very, very worrisome,” Mr Borrell told reporters on Monday as he arrived at a meeting of EU foreign ministers.
“We are facing some events, some facts, that certainly worries me as a former president of the European Parliament.”
Mr Borrell said the investigation did not target anyone from the EU’s diplomatic service.
Also speaking before Monday’s meeting, Mr Coveney said the allegations were “very damaging”, adding that “we need to get to the bottom of it”.
Meanwhile Terry Reintke, who co-chairs the Green bloc in parliament, has called for Ms Kaili’s immediate resignation.
“We shouldn’t even have to remove her from her vice president position,” the German MEP said. “This should be done by her proactively.”
Prosecutors said they suspected a Gulf state had been influencing economic and political decisions of the parliament for several months, especially by targeting aides.
Local media has named the state as Qatar, though the Qatari government said any claims of misconduct were “gravely misinformed”.
Ms Kaili’s responsibilities as vice-president include the Middle East. She has been a defender of Qatar in the past.
Ms Metsola flew from her native Malta to Brussels on Saturday evening to witness the searching of an MEP’s house – as is required by the Belgian constitution.
Her spokesman said the European Parliament “stands firmly against corruption” and is “fully cooperating” with investigators.
The European Parliament is the EU’s only directly-elected institution. Some 705 members of parliament, elected by voters in the 27 nations which make up the EU, meet to scrutinise proposed legislation and vote through European law.
