Gaza militants fired a rocket into southern Israel on Thursday, causing no injuries but undermining a shaky, week-old truce meant to halt a violent cycle of attacks and harsh Israeli reprisals.

It was the second breach of the Egyptian-mediated truce by Palestinian militants. The Israeli military office said the rocket landed in an open field on a communal farm.

The Israeli government had no immediate response to the latest rocket fire, but security officials said Defense Minister Ehud Barak convened a meeting of security officials to decide how to respond.

Skepticism about the truce's ability to hold was widespread even before it took effect June 19.

The initial objective of the deal was to halt the rockets and mortars that have bombarded southern Israel for years and ease Israel's bruising blockade of Gaza.

Israel resealed its cargo crossings with Gaza on Tuesday after militants fired three rockets into southern Israel. The passages have remained closed since then, cutting off shipments of basic supplies that had been increased slightly as part of the cease-fire deal.

Before the rocket attack Thursday, Hamas spokesman Sami Abu Zuhri denounced the resealing of the border as a "severe breach of the calm agreement."

Hamas, which has ruled Gaza for the past year, has said it will enforce the truce, but not confront militants from other groups who violate the deal.

The Al Aqsa Martyrs' Brigades, a violent group linked to the rival Fatah movement, claimed responsibility for Thursday's attack. In a text message sent to reporters, it said "the truce must include the West Bank and all sorts of aggression must stop."

The first truce infraction took place after Israel killed two Palestinians in a raid in the West Bank city of Nablus, including a militant leader. During the truce talks, Israel resisted militants' demands to extend the cease-fire to the West Bank.

The West Bank is ruled by moderate Palestinian President Mohammed Abbas, who lost control of Gaza to the violent Islamic militant Hamas a year ago. Israel is engaged in peace talks with Abbas, but carries out raids in the West Bank because it is not satisfied with the crackdown on militant groups there.

The West Bank and Gaza lie on opposite sides of Israel.

The Gaza cease-fire is meant to avert an Israeli invasion of the territory, which Israel evacuated in 2005 after a 38-year military occupation. Attacks on southern Israel from Gaza increased after the Israeli withdrawal and stepped up further after Hamas violently overran Gaza.

Since the Hamas takeover, more than 400 Palestinians, including dozens of civilians, and seven Israelis have been killed in tit-for-tat fighting.

The rocket assault Thursday came as an Israeli envoy headed to Egypt to meet with Egyptian officials on the final stage of the agreement _ a swap of hundreds of Palestinian prisoners for an Israeli soldier Hamas has held captive for two years. Israel has balked at Hamas' demands, saying its list of prisoners is full of people involved in deadly attacks on Israelis.

Hamas also has demanded that Israel reopen Gaza's strategic border crossing with Egypt in the final phase of the six-month truce deal.

The Rafah crossing has been sealed since the Hamas takeover, confining Gaza's 1.4 million people to the tiny seaside area and preventing them from receiving infusions of goods from Egypt. Israel has said it wouldn't open Rafah until the soldier returns home.