EL-QLAYLEH, Lebanon (AFP) - Israel shelled southern Lebanon on Saturday after a rocket slammed into its territory in a tit-for-tat exchange of fire across their tense border, sources on both sides said. Israeli rescue services said three people were injured when the rocket struck near the town of Maalot in the western Galilee region, triggering an immediate response from Israel. "The Israeli army considers this a serious incident and believes it is the responsibility of the Lebanese government and the army to prevent this rocket fire," an Israeli army spokesman said. Lebanese Prime Minister Fuad Siniora denounced the violence, which caused panic on both sides of the border, while Hezbollah denied any involvement. "The Israeli shelling is an unacceptable and unjustified violation of Lebanese sovereignty," Siniora said in a statement. "The rockets launched from Lebanon threaten the country's security and stability and constitute a violation of UN Security Council Resolution 1701." Lebanese President Michael Sleiman reiterated his opposition to the country being used as a platform for the launch of rockets, saying he regards it as a challenge to Lebanon's will. Hezbollah spokesman Ibrahim Mussawi told AFP that the group had "nothing to do" with the attack, which was launched from a region largely controlled by Hezbollah and its Amal party ally. The head of the UN peacekeeping force in Lebanon (UNIFIL), which patrols the border area urged maximum restraint. "The UNIFIL force commander Major General Claudio Graziano has been in contact with the senior commanders of the Lebanese and Israeli army with a view to ensuring that the cessation of hostilities is maintained," UNIFIL spokeswoman Yasmina Bouziane told AFP. Meanwhile, US Senator John Kerry met Syrian President Bashar al-Assad on Saturday, SANA news agency reported, as Washington reviews its policies toward countries the previous administration regarded as hostile. Kerry is the latest US legislator to visit Damascus this week. His talks with Assad were set to focus on Syria's support for Iran and regional fighters' groups, including Hezbollah in Lebanon. "We want Syria to respect the political independence of Lebanon, we want Syria to help in the process of resolving issues with Hezbollah and with the Palestinians," Kerry said on Wednesday in Lebanon. "We want Syria to help... with the disarmament of Hezbollah," added Kerry. Kerry said in Lebanon that US President Barack Obama's administration plans to adopt a fresh approach in the Middle East "but without any illusion." "Unlike the Bush administration that believed you could simply tell people what to do and walk away and wait for them to do it, we believe you have to engage in a discussion," he said. Meanwhile, two Gaza fighters were killed on Saturday in an explosion near the border with Israel, Palestinian sources said, although there were conflicting reports about the circumstances of their death. Muawiya Hassanein, the head of Gaza emergency services, said the two men were killed by the Israeli shelling and gunfire in the village of Juhr al-Dik southeast of Gaza City. An Israeli military spokesman denied there was any exchange of fire in the area but said that two mortar rounds were fired from Gaza overnight. Palestinian witnesses said the explosion occurred when the two fighters apparently tried to fire a mortar round towards Israeli territory.