Top leaders of the European Union (EU) on Friday agreed to introduce a "safeguard mechanism" for better border governance amid migratory pressures, reforming the Schengen regime that currently grants passport-free travel within 25 European countries.

"Without undermining this basic principle [of free movement of persons], we felt the need to improve the Schengen rules," European Council President Herman Van Rompuy said at a press conference as EU leaders wrapped up their two-day summit in Brussels.

Under this mechanism, a clause could be introduced to allow the exceptional re-introduction of internal border controls in a truly situation, "when a member state is no longer able to comply with its obligation," Van Rompuy said.

The European Commission, the EU's executive arm, will present its proposals on the mechanism by September, its president Jose Manuel Barroso said at the same conference Friday.

"The summit has made some crucial decisions with substantial consequences...The commission will come forward with concrete proposals shortly," Barroso said.

Referred to as "free movement of the persons" and hailed as EU's hallmark achievement, the Schengen system has been under huge pressure following a sudden influx of migrants from North African countries in the spring.

The idea of re-introducing internal border controls was first mooted by Italy and France, after which the commission worked out a series of policy initiatives.

EU interior ministers also agreed last month on the commission's proposal of re-introducing temporary internal border controls "as a last resort" and "in truly critical conditions," along with a package of other measures for long-term border governance.

The move is deemed by many as an overhaul of the 16-year-old Schengen zone that now includes 22 participating EU countries plus Norway, Iceland and Switzerland.