The European Union on Monday ruled out changing Europe's passport-free system of travel despite mounting pressure over a thwarted attack on a multinational rail line and a large influx of migrants into the EU.

The so-called Schengen agreement allows travel without border checks between 22 EU member countries plus Iceland, Liechtenstein, Norway and Switzerland.

EU Commission spokesman Christian Wigand said that "Schengen is non-negotiable and the Commission has no intention of changing it."

But he said that Schengen rules allow for security to be stepped up by national police as long as actions are targeted and do not substitute for border checks. Countries can also temporarily close their borders in exceptional circumstances.

Unlike airports, international trains in Europe do not have routine identity and baggage checks, apart from the Eurostar train linking the continent to Britain, which is not a Schengen member.

Belgium's prime minister said over the weekend that the rulebook might have to be re-examined if police are not able to provide sufficient security for travelers.

Germany has called for measures to prevent migrants traveling unchecked through Europe, and French police bolstered checks on the border with Italy in June as hundreds of migrants waited to cross, raising concerns at EU headquarters in Brussels. Austria and Hungary have also beefed up border security.

The Commission set up a rail security working group in 2012 but it made little progress due to a lack of interest among members.

EU transport spokesman Jakub Adamowicz said the forum could now be used to discuss possible new security measures ahead of a meeting of European transport ministers on October 7-8.

But Adamowicz warned against "any over-reaction that could be counter-productive," and said possible new measures must be proportionate to the security threat.