CAIRO- Egypt's top appeal court overturned the jail sentences of 16 NGO workers and ordered their re-trial in a case that has been a point of tension in U.S.-Egypt relations, state news agency MENA reported on Thursday.

In 2013 an Egyptian court sentenced 43 Americans, Europeans, Egyptians and other Arabs to jail terms ranging from one to five years and ordered their non-governmental organisations (NGO) closed, following their conviction on charges including operating without necessary approvals.

Many of the defendants, including at least 15 Americans, left the country and received five-year sentences in absentia, with those remaining receiving one and two-year jail sentences.

The groups are associated with democracy promotion and include the U.S.-based International Republican Institute (IRI), National Democratic Institute (NDI) and Freedom House.

The 16 defendants had been charged with operating without necessary approvals and receiving funds from abroad illegally, but some jail sentences were previously suspended as the case was the subject of appeal.

Following Thursday's ruling by the Court of Cassation, the defendants will now have their case re-tried by a Cairo criminal court, MENA reported.

Egyptian rights activists say they are facing the worst assault in their history since President Abdel Fattah al-Sisi came to power in 2014 amid a campaign to erase the freedoms won in the 2011 uprising that ended Hosni Mubarak’s 30-year rule.

The case, as well as a law restricting NGO activity that has yet to be ratified by Sisi, has strained Egypt-U.S. relations, with the United States last year withholding about $100 million in aid and saying it would delay a further $200 million over concerns tied to human rights.

(Reporting by Haitham Ahmed, Writing by Eric Knecht, Editing by Giles Elgood and William Maclean) ((mailto:eric.knecht@thomsonreuters.com; +20 2 2394 8102; Reuters Messaging: rm://eric.knecht.thomsonreuters.com@reuters.net))