Belgium was hit by a national strike on Monday, with air and rail traffic severely affected as workers protested against pension reform plans.

About 40 per cent of flights taking off or landing at Brussels airport - Belgium's largest - were cancelled, as baggage handlers, security staff and other personnel downed tools, the company operating the terminal told AFP.

Flag carrier Brussels Airlines said on Saturday it was forced to pre-emptively cancel half its flights within Europe to keep long-haul routes running.

The operator of Charleroi airport, a major European hub for low-cost airline Ryanair, said significant delays were expected and cancellations possible. 

Railways saw a spate of cancellations, with two in three trains connecting major cities not operating. 

Service was also disrupted in Brussels and the southern French-speaking Wallonia region. 

Many teachers, particularly in the Dutch-speaking northern Flanders region, where federal authorities have a say over education, joined the walkout, according to the unions.

The nationwide strike was called by Belgium's three main unions to protest against pension cuts that might be enacted by the incoming government. 

Belgian parties are still in talks to form a new federal administration following elections in June.

The five parties negotiating to strike a coalition deal plan to raise the retirement age for railway workers and others who are now allowed to retire early, according to CSC, a trade union. 

Overall "there are at least three billion (euros) in savings planned on pensions", CSC general secretary Marie-Helene Ska told local radio RTBF.

The unions plan to hold a demonstration in Brussels during the day.