A young Israeli woman convicted of spying and leaking classified military documents to Israel's Haaretz newspaper began her four-and-a-half year prison sentence on Wednesday.

Wearing a red sweat shirt, grey sweat pants and dark glasses and carrying a large holdall, former soldier Anat Kam, 24, entered Neve Tirza women's prison in Ramle near Tel Aviv, an AFP correspondent said.

She was convicted in February under a plea bargain which saw her admitting to charges of "serious espionage" and "passing on classified data" in return for the prosecution dropping a charge of intent to harm state security, which carries a maximum sentence of life imprisonment.

Last month, she was sentenced to 54 months behind bars with an additional 18 months suspended for three years from completion of the term.

Although she pleaded guilty to the offences, she is appealing the length of the sentence which her lawyer, Ilan Bombach, says is excessive and fails to take into account the time she spent under house arrest since December 2009.

"It appears there is a very reasonable chance that the sentence will be significantly shortened," Bombach told public radio.

"In the past few days we have submitted an urgent request for the appeal to be heard with all possible speed so that she does not sit behind bars any longer than necessary."

Kam was convicted of stealing more than 2,000 military documents, including details of operational planning and force deployment, during her army service between 2005 and 2007.

Some of the documents were used as source material for a Haaretz article which said troops had been ordered to carry out targeted killings of Palestinian militants in violation of an Israeli Supreme Court order.

Uri Blau, the journalist who wrote it, returned the classified documents in his possession and has so far not been charged, with his lawyer saying a decision was pending.

"There was a hearing by the attorney general and there is still no decision," Mibi Moser told AFP.

Kam said her actions were ideologically motivated and that she wanted to expose the army's policies in the Palestinian territories.