BRUSSELS, July 18, 2011 (AFP) - The European Commission warned on Monday that Denmark has failed to justify a controversial decision to increase controls at its borders with Germany and Sweden.

The European Union's executive arm said experts met with Danish authorities last Thursday and visited the country's borders with its two European neighbours, which are part of the Schengen passport-free travel zone.

"Unfortunately, the mission did not enable us to get adequate answers to our questions," said EU home affairs commissioner Cecilia Malmstroem.

"There are persistent concerns about the compatibility of Denmark's strengthened internal control measures with the freedoms provided under the EU Treaty including the Schengen acquis," she said.

"It is incumbent on Denmark to demonstrate factually that the gravity of the situation justifies putting in place controls which might affect the exercise of free movement of goods, services and persons at the internal borders with Germany and Sweden."

The result of the mission makes it "even more necessary to establish a reinforced dialogue with the Danish authorities and to put in place a strict monitoring system based on regular information from the Danish authorities."

A letter was sent to Danish authorities to get further clarifications and further visits are not excluded, she said.

"The commission will not hesitate to use all tools at its disposal to guarantee free movement of goods, services and persons and the full respect of EU legislation," she said.

Denmark deployed 50 new customs officers at its borders with Germany and Sweden on July 5, a controversial measure hammered out under pressure from its far-right ally which drew critics from Brussels and its neighbours.

The Scandinavian country argues random border checks are in line with Schengen and that their aim is to combat the smuggling of illegal goods and drugs, not to control travellers.

"In a first assessment the experts reported that they were unable to get sufficient justifications from the Danish side for the intensification of the controls at the internal borders," the commission said in a statement.

"In particular, according to the experts, the risk assessment required to justify the controls was not sufficient and there were no clear instructions to border control officers on how to carry out controls," it said.

"There also does not seem to be a structured reporting mechanism about the numbers of controls and the results of the intensified border checks."