CAIRO (Reuters) - Egypt on Monday said it had detained an Israeli army officer in the Taba region of the Sinai peninsula for sneaking through the border, but his mother and Israeli media said he was a civilian pro-Palestinian activist.

The Egyptian state news agency MENA said the man was a Tel Aviv resident of Russian origin who was not carrying a passport, and that his name had not been on tourist arrival lists. An Egyptian state security officer said the man was an army officer who had been detained on Saturday and was still being interrogated.

But Israeli media reported that Andrei Pshenichnikov, a known pro-Palestinian activist, had crossed into Egypt with the intention of entering the Gaza Strip, an area that is off limits to Israelis for security reasons.

His mother, Svetlana, told Israel Radio that he was in custody in Egypt and that he had intended to travel via Sinai to Cairo to rendezvous with friends from France.

"He received his visa (to visit Egypt) and went to Eilat, intending to cross into Egypt, tour the area and then go to Cairo, but Israeli police stopped him at the border and said he had tried to cross the border illegally ... They held him for several days and they demanded he sign an undertaking not to go to Cairo, but he refused."

She added that her son was eventually released by Israeli authorities after his Israeli and Russian passports had been confiscated, but that later in the day he had called from Taba and said he had been detained there by Egyptian authorities.

"After he finished his army service ... he supported (Palestinians) ... He later rented an apartment in a refugee camp in Bethlehem to prove to the locals that there are Israelis who are in favor of peace," Svetlana added, in poor Hebrew with a heavy Russian accent.

Egyptian security sources in Sinai said the detainee had been gathering information about Sinai from drivers in the area near the Taba border crossing between Egypt and Israel.

Egypt is trying to reassert control over Sinai, which has suffered from lax security since the uprising that overthrew Hosni Mubarak in February 2011.

(Reporting by Yousri Mohamed and Marwa Awad; Writing by Shaimaa Fayed and Ori Lewis in Jerusalem; Editing by Kevin Liffey)