JERUSALEM -- Shortly after he was seized by Palestinian militants, then-Cpl. Gilad Shalit wrote of his ordeal as an "intolerable and inhumane nightmare." The letter, one of just three that the captive Israel soldier has been allowed to write, appealed to authorities to bring him home from his "closed and solitary prison" in the Gaza Strip.

Yesterday, after more than three years of indirect negotiations for his freedom, Israel and Gaza's Hamas rulers reported the first tentative step toward a deal -- the exchange of 20 female Palestinian prisoners for a recent videotape as proof of now-Staff Sgt. Shalit's well-being.

The swap, scheduled for tomorrow, was described in Israel's announcement as a confidence-building measure in advance of "the decisive stages" of talks aimed at trading the 23-year-old conscript for a far-larger number of Palestinian militants.

Israel has conducted prisoner exchanges before. But this is believed to be the first time during its decades of armed conflicts with Arab neighbors that the Jewish state has agreed to trade imprisoned adversaries for information about an Israeli captive.

The negotiations ahead are expected to be difficult, fraught with the same obstacles that have long frustrated Egyptian mediators. Among the 1,000 or so prisoners whom Hamas wants freed are some Israel has insisted on keeping locked up because it alleges they made deadly attacks against its citizens.

"This is a positive step in the negotiations," Israeli President Shimon Peres said after yesterday's announcement. "But the road to his release is still long and not simple, and we do not want to create any illusions."

Israel and Hamas nonetheless have reasons for wanting to strike a deal.

Sgt. Shalit's homecoming would end a painful ordeal for Israel, a country where military service is mandatory for Jews. Israelis have rallied behind the soldier and his family, which has led a vocal campaign to get him freed.

For Hamas, Sgt. Shalit's release would satisfy a key Israeli condition for ending the economic blockade that has caused shortages of many basic items for Gaza's 1.5 million people and blocked repairs of the extensive damage caused by Israel's 22-day military offensive last winter. The restrictions were tightened after the soldier's capture, then tightened again when Hamas took control of Gaza in 2007.

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