The spokesman would not provide details about the violation, but an Associated Press photographer who witnessed Thursday's incident said an Israeli armoured vehicle and two jeeps drove through the border fence and tried to penetrate further into Lebanese territory when UN French peacekeepers blocked their path.
The incident came six weeks after a ceasefire between Israel and Hezbollah fighters went into effect in the region, and happened near the Lebanese border village of Marwaheen.
It ended peacefully after a standoff that lasted about half an hour, the photographer said.
The Israeli army confirmed that its troops came close to the French peacekeepers on Thursday but said there was no conflict between the two sides.
Israeli forces have been operating inside Lebanon in the Marwaheen area since the ceasefire took effect and would continue to operate there until it is transferred over to international peacekeepers, the army said.
Alexander Ivanko, a spokesman for the UN peacekeeping force known as Unifil, would not provide details about the violation but also said there was no standoff.
"The French peacekeepers observed an Israeli violation of the Blue Line and reported it to the Unifil command," Ivanko said, referring to the UN-demarcated line between Lebanon and Israel.
The UN peacekeeping force began deploying in southern Lebanon last month to cement the UN-brokered ceasefire between Israel and Hezbollah fighters.
Daily protests by supporters of
the Hezbollah worry Israel
Ivanko said the UN expects the Israeli withdrawal from southern Lebanon to be completed by the end of the month. But Israeli officials said on Thursday that they were reluctant to complete the pullout.
The AP photographer said he saw French peacekeepers talk with Israeli soldiers after UN tanks blocked the Israeli vehicle and jeeps from driving further into Lebanon. After a peaceful standoff that lasted about a half hour, the Israeli armoured vehicle and the two jeeps moved back into Israel, and UN peacekeepers remained at the site, the photographer said.
Despite repeated Israeli assurances of a quick troop withdrawal from Lebanon, Israeli officials say they are reluctant to complete the pullout.
Several issues - large and small - remain unresolved, involving the future roles of Hezbollah fighters, UN forces and the Lebanese army in the border area, and the overall prospect of keeping the guns and rockets silent.
Israelis also warn that daily protests on the border, in which a few dozen people waving large Hezbollah flags throw rocks at Israeli army vehicles patrolling a road in Israel across the chain-link fence, could escalate into cross-border fighting.
Cabinet Minister Gideon Ezra told a cabinet meeting on Wednesday that Israel had to stop the protests before they spiral out of control. The Israeli army chief, Lieutenant-General Dan Halutz, told the meeting he instructed soldiers to fire on stone-throwers in self defence and informed the UN forces of his decision.
"We clarified that we will use means to disperse protests against Hezbollah along the border," Halutz told the cabinet, according to a participant. Soldiers who feel threatened will fire in the air and then at the legs of demonstrators, Halutz said.
Israeli security officials say a few thousand Israeli troops are still just across the border in Lebanon, left over from a large-scale Israeli offensive that followed a July 12 cross-border raid by Hezbollah fighters, who killed three soldiers and captured two others.
The Israeli withdrawal from Lebanon is a key component of the UN ceasefire that took effect August 14 and ended the 34 days of fighting between Israel and Hezbollah. More than 850 Lebanese were killed, mostly civilians, and almost 160 Israelis died in the war.
The truce is outlined in a UN Security Council resolution, which calls for an international force of 15,000 UN peacekeeping troops to join 15,000 Lebanese soldiers in patrolling a buffer zone in southern Lebanon to prevent renewed hostilities between Israel and Hezbollah.