Around 200 flights were cancelled at Frankfurt Airport on Tuesday as ground workers announced they would extend their four day strike until Friday.

The union representing the workers said it was extending its strike to press its demands for more pay from airport operator Fraport AG.

The strikes that began last week have already led to the cancellation of hundreds of flights and delays at the airport, Europe's third busiest, which sees some 1,300 takeoffs and landings daily.

Fraport says the union's demands for double-digit wage increases are "absolutely unacceptable" while the union says it just wants the Frankfurt workers' salaries brought in line with what other airports pay.

The union and Fraport have been unable to come to a new agreement after weeks of negotiations.