It was not immediately clear whether the foray was the start of a larger Israeli operation in northern Gaza.
It came several hours after Palestinian militants fired four Qassam rockets from northern Gaza toward Israel. One rocket hit in an industrial zone a half-mile outside the Israeli coastal city of Ashkelon, the deepest strike since Hamas began firing Qassams toward Israel in November 2001. It caused no damage or casualties.
Palestinian police said they traded fire with the militants, but the militants escaped.
After Thursday's strike, Israeli officials threatened action to prevent the rockets from being fired.
The primitive Qassam rockets rarely cause damage or injuries, but Israel considers them a strategic threat. Tanks were sent to the border with Gaza several days ago, poised for the possible retaking of Palestinian-controlled areas of Gaza if shelling continues.
"We will have to act with all means at our disposal to prevent these acts in the future," said Zalman Shoval, an Israeli government spokesman.
Israel has said it will keep hunting militants until the Palestinian Authority begins dismantling the armed groups, as required by the U.S.-backed "road map" peace plan.
Meanwhile, the Palestinian Authority has apparently frozen 39 bank accounts of nine Islamic charities, according to an official document obtained Thursday by The Associated Press.
Israel welcomed the freezing of the funds, which came as the Palestinian Authority has been under growing U.S. pressure to take action against militants, including Hamas and Islamic Jihad. Those groups formally abandoned a unilateral truce last week after Israel killed a Hamas leader in response to a deadly Hamas bus bombing.
Despite U.S. prodding, the Palestinian government has been reluctant to arrest militants and seize their weapons because of concern about triggering unrest and because of wrangling between Yasser Arafat and his prime minister, Mahmoud Abbas, over control of the security forces. Palestinian police have sealed several tunnels used for smuggling weapons and drugs from Egypt into Gaza.
Also Thursday, Israeli soldiers in Nablus fired rubber bullets and live rounds at stone-throwing teenagers. At least nine Palestinians were wounded, three of them seriously, hospital officials said. The army said that in two cases, Palestinians shoved ovens from ledges or roofs, trying to hit soldiers.
Dozens of Palestinians also threw rocks at Israeli tanks in Jenin, but no injuries were reported.
In the nearby town of Qabatiya, soldiers blew up the two-story house of an Islamic Jihad militant who died in a Jan. 1 shooting attack after killing an Israeli. Witnesses said 15 relatives left the house before it was demolished, a common action by the army intended to dissuade others from attacks.