Under Germany's new military service law, men aged between 17 and 45 will have to seek approval for being abroad for more than three months, the Defence Ministry confirmed to dpa on Saturday following press reports.

Chancellor Friedrich Merz's administration passed a law late last year to bring back military service - initially in a voluntary capacity - in an effort to boost troop numbers.

The law, which took effect on January 1, is a response to the threat posed by Russia, with Germany lagging in its NATO recruitment targets.

The programme is intended to assist in raising troop levels in the Bundeswehr - Germany's military - from 180,000 to 260,000, with an additional reserve force due to reach 200,000.

All teenagers are to receive a questionnaire after turning 18 as a first step to assess their suitability and motivation to serve in the military. Men are required to fill out the form, while it is voluntary for women.

The Frankfurter Rundschau newspaper has reported that men require approval from the Bundeswehr for lengthy trips abrod.

A Defence Ministry spokesman confirmed the regulation, but said that "approval is taken as given as long as the service is voluntary."

The military needs to know who is spending long periods abroad, the spokesman said, but he declined to say how many approvals had been requested since the start of the year.

He further noted that the rule had been in force since the Cold War but had never been strictly enforced.

While the consequences of the rule were far-reaching, permission for stays abroad would always be granted if no specific service was required for the relevant period, he said.