An Israeli aircraft on Monday fired at a car carrying a group of Palestinian militants in the northern Gaza Strip, wounding one person, shortly after Prime Minister Ehud Olmert warned that he was losing patience with ongoing rocket barrages on southern Israel.

The violence raised new doubts about the future of a six-month-old cease-fire in Gaza. The truce has largely held, despite periodic flare-ups in violence.

At a meeting with the visiting German foreign minister Monday, Olmert said the Palestinians would face severe consequences if the latest wave of rocket fire persists.

"The (rocket) attacks constitute a tangible threat to Israel. Israel cannot show restraint forever," Olmert said, according to a statement issued by his office.

Despite the truce declared last November, militants in Gaza have continued to fire the homemade rockets into southern Israel, though fewer than before then. But in recent days, the rocket fire has increased.

The army said 14 rockets have been fired since Friday. One rocket hit a house early Monday, damaging the building. There were no injuries.

In Monday's airstrike, the army said it attacked an explosives-laden vehicle carrying militants preparing an attack. The car was driving near Beit Hanoun, a frequent launching ground for rocket squads.

The Islamic Jihad militant group said its members were in the car on a "holy mission." It said the men escaped the blast, and that a passer-by was wounded. Medical officials said the man, whose identity could not immediately be confirmed, was in moderate condition.

Palestinian President Mahmoud Abbas has repeatedly condemned the rocket attacks, but has been unable to halt them. Ending the rocket fire is a key component of a new U.S. proposal for easing Israeli restrictions on Palestinian movement while also improving Israeli security.

The document proposes a May-to-August timeline that calls for Israel to remove many West Bank roadblocks and improve operations at Gaza's crossings.The Palestinians are asked to halt rocket fire from Gaza and weapons smuggling into the coastal strip.

In a meeting with his Fatah Party, Abbas urged Israel to cooperate with the plan. The document is "a start to end the suffering of the Palestinian people," he said, the official WAFA news agency reported.

In Jerusalem, Olmert said he is still considering the American proposal and will finalize his position shortly. He noted that Israel already has committed to some of the proposals, such as improving movement through Gaza's border crossings.

It remains unclear how much the U.S. document can accomplish. The Islamic Hamas, Abbas' partner in the Palestinian coalition government, has rejected the document.

Israel has grown increasingly concerned by arms smuggling into the Gaza Strip and the rocket fire. Senior military officials have called for a large-scale military operation in Gaza.

Others, however, have questioned the wisdom of sending ground troops into Gaza's crowded urban landscape. Similar operations since Israel withdrew from Gaza in 2005 have failed to halt the rocket fire.

On Monday, Israeli Foreign Minister Tzipi Livni said Israel should not rush into a military operation in Gaza.

"Gaza is turning into a terror nest. We know all this," Livni said. "It is the government's responsibility to hold an intelligent and comprehensive decision process. "

She said Israel wants to make progress in peace talks with the Palestinians, despite her government's troubles following an explosive report on last year's handling of the war in Lebanon.

The report, which criticized Olmert for "very severe failures" during the war, has severely weakened the prime minister and caused divisions within the government. Livni last week called on him to resign following the report.

Livni acknowledged that it would be difficult for the government to act in the "crisis of trust" now between its leaders, but said she would push for Israel to act on the Palestinian track.

"As I said before, stagnation is not a good policy," Livni told reporters at a news conference with German Foreign Minister Frank-Walter Steinmeier. "Even during this period we should do the right thing in order to represent the Israeli interests in peace."

The U.S. has been urging Olmert and Abbas to hold frequent meetings in hopes of jump-starting peace talks, which ended more than six years ago.

In addition, the Arab League has renewed a peace initiative that offered full ties with Israel in exchange for an Israeli withdrawal from the lands it captured in 1967. Israel has welcomed the plan as a good starting point for negotiations, but has some reservations.

Livni is scheduled to travel to Cairo on Thursday to discuss the plan with her Egyptian and Jordanian counterparts.