BERLIN (Reuters) -Human rights lawyers have filed a court appeal in Berlin seeking to block a 150-metric-ton shipment of military-grade explosives aboard German cargo ship MV Kathrin which they say is to be delivered to Israel's biggest defence contractor.

The European Legal Support Center (ELSC) said on Wednesday the action was filed on behalf of three Palestinians from Gaza, arguing that the shipment of primarily RDX explosives could be used in munitions for Israel's war in Gaza, potentially contributing to alleged war crimes and crimes against humanity.

Israel denies accusations that it has committed war crimes in the Gaza Strip, saying its forces abide by international humanitarian law while fighting Palestinian militants who operate in densely populated civilian areas.

Germany-based Lubeca Marine, which owns the MV Kathrin, said the ship "was never scheduled to make any port calls in Israel" and had recently discharged its cargo in Bar, Montenegro.

The company declined to disclose details of the cargo for contractual reasons, but said it complied fully with all international and EU regulations, ensuring necessary permits are obtained before any operations.

According to the ELSC, the RDX is destined for Israeli Military Industries, a division of Elbit Systems, Israel's largest defence contractor. Elbit Systems declined to comment.

"We never claimed that the Kathrin was bound for Israel (itself), it's the cargo which is bound for Elbit Systems," ELSC lawyer Ahmed Abed told Reuters with respect to the group's appeal filed at Berlin's Administrative Court.

"The company ignored all the warnings."

The ship has been denied entry at several African and Mediterranean ports, including in Angola, Slovenia, Montenegro, and Malta, according to the ELSC.

It said Portuguese authorities recently required the ship to switch from a Portuguese flag to a German flag before it could continue.

In August, Namibian authorities blocked the vessel, which departed from Vietnam's port of Haiphong, from entering its main harbour, Amnesty International has reported.

Germany's economy ministry, named in the case because the ship is German-owned and flagged, said it had received letters from lawyers on the matter but declined to comment on them.

The ministry said the MV Kathrin shipment did not constitute an export from Germany, as the explosives were neither loaded nor dispatched from German territory. It said there was no legal basis for requiring an export licence under German law.

(Reporting by Riham Alkousaa, additional reporting by Steven Scheer; editing by David Holmes and Mark Heinrich)

By Riham Alkousaa