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Ground staff at Berlin's two main airports have kicked off a 25-hour strike, which is expected to force the cancellation of more than 650 flights.

The disruption at the Tegel and Schoenefeld airports began at 4 am (0300 GMT) on Monday morning.

Ground crews have already walked out several times over the past few weeks amid a tense dispute over pay.

On Friday, a similar strike led to nearly 700 flights being grounded at the two airports.

Offer rejected

The service workers' union Verdi is demanding higher wages for around 2,000 workers handling passengers and baggage.

The two airports said in a statement that 448 flights would be cancelled at Tegel and 194 at Schoenefeld.

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German flag carrier Lufthansa said it would cancel all flights on Monday from Frankfurt and Munich to Berlin and vice versa. Other airlines affected include Air Berlin, British Airways, Easyjet and Ryanair.

The union is demanding a pay rise for ground staff to 12 euros per hour ($12.80, £10.50) from about 11 euros, as part of a one-year collective agreement.

Lower offers from management have been dismissed by the unions as unacceptable.

Ground staff jobs include checking in passengers, loading and unloading baggage and cargo and directing aircraft on the tarmac.