The Popular Resistance Committees (PRC) group on Thursday warned Israel that a truce between Israel and Hamas which went into effect on June 19 was in danger of collapse, saying it could end in three weeks.
Abu Mujahed, one of the group's leaders, told dozens of Gaza fighters undergoing military training that Hamas, the PRC and other factions were disappointed at Israel's slow action on opening Gaza's border crossings and prisoner release talks.
"(Israel) has until the end of the tenth week (since the declaration of the ceasefire) and if they do not abide by the obligations of calm, politicians will stop talking and military men will act," Abu Mujahed said.
He said the Palestinian groups had agreed among themselves to give the truce a 10-week trial period, although he did not say whether Hamas, which controls the Gaza Strip, was also involved in the deadline.
PRC gunmen together with groups from Hamas and another armed faction kidnapped IDF soldier Gilad Shalit in a June 2006 cross-border raid.
Israel tightened its cordon around Gaza after Hamas Islamists took control of the territory a year ago.
The Egyptian-brokered truce calls on Gaza militants to halt rocket attacks in return for Israel gradually easing an embargo on the territory. The truce has largely held and rocket attacks from Gaza have declined.
On Wednesday, a rocket fired from Gaza landed in southern Israel, but caused no injuries or damage, Israel's military said.
Israel has responded to previous attacks by reclosing border crossings with the coastal enclave.
"The Zionist occupation has not yet agreed to the demand to release our prisoners so our fighters are preparing for the next round in which we will try to abduct more Israeli soldiers to swap them for our hero prisoners," Abu Attaya, spokesman of the PRC's armed wing, told Reuters.
The Islamist Hamas, which controls the Gaza Strip, has called on Israel to release hundreds of prisoners in exchange for Shalit.