The Pentagon has suspended the delivery of a shipload of munitions to Israel after international concern that it could be used by Israeli forces in Gaza.

The German-owned cargo vessel, Wehr Elbe, under charter by the US Military Sea­lift Command, is currently in Greek waters with its transponder tracking turned off to prevent its location being identified.

Amnesty International has written to the foreign secretary, David Miliband, asking him to make "urgent approaches to the US, German and Greek governments to prevent this, or any pending or future shipments of weaponry until it can be verified that they will not be transferred to the Israeli Defence Forces or other parties to the conflict in Gaza.

"We urge you to ensure that no EU member state will allow their ports or other facilities to be used to transit these or any other weapons to any of the parties to this conflict."

The Wehr Elbe, owned by the Hamburg company Oskar Wehr, arrived outside the Greek port of Astakos on 1 January, where it was due to transfer its 1,000 containers to another vessel for delivery to Ashdod in Israel.

But after a two week stand-off, amid local protests in Greece, it moved out into the Mediterranean two days ago and disappeared off tracking websites.

Lieutenant Colonel Patrick Ryder, a Pentagon spokesman, said that the contract for the munitions had been arranged last summer and approved in October. He said the munitions were due to be delivered to a US pre-positioning depot in Israel for US forces. But he added: "If the government of Israel requests munitions they can do so direct to the US government under the Foreign Military Sales programme."

He said the ship's journey had been delayed due to "safety concerns" about unloading the cargo at Ashdod and that other arrangements were being made by the Military Sealift Command's European office in Naples.

The letter to Miliband, from Amnesty's director, Kate Allen, calls "for a comprehensive arms embargo on Israel, Hamas and Palestinian armed groups until effective mechanisms are in place to ensure that weapons and munitions and other military equipment will not be used to commit serious violations of international humanitarian law".

Amnesty's Middle East and North Africa programme director, Malcolm Smart, said: "The last thing that is needed now is more weapons and munitions in the region, which is awash with arms that are being used in a manner which contravenes international law and is having a devastating effect on the civilian population in Gaza."