Residents of towns along the Gaza border have been instructed to remain vigilant in case Hamas tries to infiltrate the communities to carry out terrorist and kidnapping attacks. According to defense officials, the tension along the border is at one of its highest levels since Operation Cast Lead ended on January 18. The warnings collected by the Shin Bet (Israel Security Agency) focus mainly on terrorist threats along the security fence, as well as infiltrations into Gaza-belt communities. While Hamas has ceased its rocket attacks and is trying to curb other factions and prevent them from firing into Israel, the group is still planning larger-scale attacks along the border, officials said. In June, a major terrorist attack that included 10 gunmen and explosives-laden horses was thwarted by the IDF near the Karni crossing. The attack was carried out by a group affiliated with al-Qaida. On Monday, a mortar shell struck near a group of IDF engineers working on the security fence. The day before, two mortar shells and a Kassam rocket were fired into Israel. The shells hit near the Erez crossing, meters away from ambulances that were transferring a Palestinian heart patient to Israel for treatment. In response to Sunday's attack, the IAF bombed a smuggling tunnel under the Egyptian-Gaza border. The army said the air strike was in response to a Kassam rocket that hit an open field near the Sdot Negev region earlier that day. According to a senior officer in the IDF Southern Command, troops deployed along the Gaza border have also been facing an increased spate of sniper attacks. In addition, there has been an increase in the number of bombs planted along the security fence. Besides to the threat of infiltration by tunnel or over the fence, there is also growing concern in the army that terrorists will try to enter Israel by sea, either by swimming or in rubber boats. The Palestinians' use of snipers poses an operational challenge for the IDF, the senior officer said, since the gunmen fire at the troops from more than half a kilometer away. "They fire from 500 to 600 meters away," the officer said. "This makes it difficult to then locate the sniper and find an available platform to hit him with."