A second round of strikes has left thousands of commuters stranded as Lufthansa Airlines has been forced to cancel over 200 flights in and out Germany.Tuesday's cancellations at the German flag carrier's Frankfurt hub came as a union chief threatened to hit every airport in Germany with a 24-hour strike on Friday.Lufthansa, Germany's largest airline cancelled short and medium-haul flights but also services to and from Los Angeles, Houston, Chicago, Beijing and Mexico City.The Unabhaengige Flugbegleiter Organisation (UFO) union and Lufthansa are arguing over pay and working conditions.Tuesday's groundings come as part of the extension of a cabin crew strike in Frankfurt last week that has now spread to Munich and Berlin.Short and medium-haul flights were the worst affected.The dispute appeared to escalate after UFO chief Nicoley Baublies threatened industrial action by the Lufthansa staff across Germany if the airline failed to compromise. Lufthansa responded by saying that it was considering taking legal action against the union.In addition to the cancellations, reduced capacity at Frankfurt, Europe's third busiest airport, led to approximately 80 further delays, said a spokesman for Brussels-based Eurocontrol, a multinational air navigation safety body."Flights which are delayed are experiencing an average of 20 minutes delay, although some flights earlier today had up to an hour," a Eurocontrol statement said.The UFO, which held a first strike at Frankfurt airport on Friday, staged fresh walkouts at airports in Berlin and Munich as well as Frankfurt with the stoppages lasting eight hours.The strikes ended at 1200 GMT in Berlin and Frankfurt but were due to last until 2200 GMT in the southern city of Munich.Disruption was less severe in Berlin and Munich, with 15 out of 39 flights cancelled in the capital and three-quarters of the flights in the Bavarian city expected to run, according to Lufthansa.A spokesman for Austrian Airlines, owned by Lufthansa but not affected by the industrial action, said the airline flew larger planes to the three affected airports to help with stranded passengers.AccusationsLufthansa and UFO traded verbal blows early on Tuesday, each accusing the other of arrogance.Lufthansa had shown "no sign of bending", Dirk Vogelsang, who is leading the negotiations, told the AFP news agency, adding that "at the moment, it looks very, very difficult" with the attitude hardening on both sides.If management continued what he termed "sabre-rattling", the union would consider "blanket strikes," he said.UFO is seeking a five per cent pay increase for cabin staff backdated to January after three years of wage freezes. It is also opposed to the use of temporary cabin staff on Lufthansa aircraft.