The European Commission is in talks with the Maltese government about its controversial sale of EU citizenship to rich foreigners under a new scheme.
The Commission says citizenship rules are a national prerogative, but it has raised concerns, as have many MEPs.
Last week EU Justice Commissioner Viviane Reding said "you cannot put a price tag on EU citizenship".
Foreigners will be able to buy Maltese passports for 1.15m euros (£944,000), without having to live in Malta.
A spokeswoman for Ms Reding said "we are in talks, but it does not mean launching infringement proceedings [against Malta]".
"It is always the case that we give a member state a chance to outline, clarify its position and respond to Commission concerns."
The spokeswoman said Ms Reding's primary concern was that an applicant for citizenship should have "a genuine link to the country" - not just the ability to pay.
"We are not prescribing - we are open to clarifications from the Maltese authorities," she told the BBC.
Malta, like most of the EU's 28 countries, is in the Schengen zone, where citizens can mostly travel without passport checks. The EU single market has made it much easier for citizens to settle in another member state.
Owning an EU member state's passport entitles the holder to EU citizenship, with all the rights guaranteed under EU law.
Under the new scheme, called the Individual Investor Programme (IIP), Malta will initially issue passports to 1,800 people. Family members will also be able to get passports, for a lower fee.
The scheme is being managed by a Jersey-based company, Henley and Partners. On its website the company says applicants will be subject to strict vetting and "only highly respectable clients will be admitted". The scheme is aimed at "ultra-high net worth individuals and families worldwide".
The original price tag for Maltese citizenship, set last November, was 650,000 euros. But the government later raised it to 1.15m euros, amid criticism of the scheme by opposition MPs.
MEPs speak out
In a strongly-worded resolution last week, the European Parliament also criticised the Maltese scheme, saying "EU citizenship should never become a tradable commodity".
The MEPs urged Malta to "bring its current citizenship scheme in line with the EU's values" and called on the Commission to "issue recommendations in order to prevent such schemes from undermining the values that the EU has been built upon".
The MEPs also questioned whether Malta was complying with Article 4.3 of the Treaty on European Union, which enshrines "the principle of sincere co-operation" between member states, who are obliged to assist each other and avoid any measure that jeopardises the EU's goals.
Several EU states, including Austria, Spain, the UK, Bulgaria and Hungary, offer fast-track residence to foreigners able to invest large sums in property and/or government bonds. A permanent residence permit can often lead to citizenship.