Workers at some of Germany's busiest ports have shocked international shipping companies and exporters by participating in last-minute strikes that have left cargo stuck on stranded vessels.

The strikes began at Germany's largest port by volume, the Port of Hamburg, at 6:30 am on Tuesday, where workers walked off the job and said they would not return until Thursday morning.

Employees at Bremen's Bremerhaven port joined in on Tuesday afternoon, also saying they would not return before Thursday morning. And workers at the Port of Emden said on Tuesday their facility would also be closed throughout Wednesday.

The strikes were part of industrial action staged by the United Services Trade Union, which is known as Verdi. The union is trying to put pressure on the Central Association of German Seaport Operators, or ZDS, as part of its dispute with the operators over pay and conditions.

Verdi is set to sit down with ZDS on Thursday and Friday for the previously scheduled fourth round of protracted negotiations for a new collective agreement for Germany's 11,500 port workers.

The German broadcaster and news agency Deutsche Welle said Verdi called for Tuesday and Wednesday's so-called warning strikes because it had become frustrated with the talks, which began in May.

Employers, the union said, had "so far only presented an inadequate offer".

The latest round of strikes followed similar industrial action during June.

Verdi is seeking "significantly better wages, especially for the lower pay groups", with a pay rise of 3 euros ($3.20) an hour and an increase in workers' shift allowances called for.

Maren Ulbrich, Verdi's chief negotiator, told DW: "In the third round of negotiations, we were still far apart. The offer presented by the employers is not acceptable to us. The employers still have to make some progress, particularly on the wage increases offered."

ZDS responded by saying it viewed negotiations with the union up to now as having been "intense but constructive" and said this week's warning strikes were not necessary and undermined progress already made.

"The right to strike is enshrined in Germany's constitution. However, when exercising the right to strike, moderation and balance should be maintained," ZDS said in a statement. "In light of the constructive rounds of negotiations to date and the fair offer presented, the ZDS believes there is currently no reason for warning strikes that would compromise the reliability of German seaports."

The German news agency Deutsche Presse-Agentur said dock workers at the Port of Wilhelmshaven were also poised to join the ongoing industrial action.

It said workers from all ports taking part in the strikes were also planning to gather outside the ZDS headquarters in Hamburg, to stage a rally and protest march.

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