Image caption A heavy military presence is building up in al-Arish 
 
 Egypt has deployed extra troops, tanks and other armoured vehicles to the Sinai peninsula in an escalation of its military offensive in the area. 
 
 The reinforcements follow the killing of 16 border guards by suspected Islamist militants on Sunday. 
 
 The army has also begun sealing off tunnels into Gaza, according to Egypt's Mena news agency. 
 
 Earlier reports that fresh fighting had broken out in al-Arish have been denied by a source quoted by the agency. 
 
 According to Nile News TV, violent clashes had broken out between police and armed men outside a police station in the town. 
 
 At the scene At least a dozen military transporters, each carrying two armoured personnel carriers, passed us on the main coastal road in al-Arish. We watched as they unloaded at the main military base here. Sights like this have not been seen in the Sinai Peninsula since Egypt's 1973 war with Israel and a small crowd of locals stood by staring. One man told us he hoped the army would come down hard on Islamist militants, describing the killing of 16 Egyptian border guards last weekend as "heinous". Others in al-Arish have expressed more cautious opinions, pointing out that in the past there has been a heavy-handed approach from security forces when dealing with the local Bedouin population. Tensions are likely to rise as action is taken to destroy the smuggling tunnels between Egypt and the Gaza Strip. There are said to be hundreds of them used mainly to transfer fuel and goods into Gaza, but sometimes to move weapons and people illegally. After years with little economic development, many North Sinai residents rely on the tunnels trade. 
 
 But a security source told Mena the shots had been fired into the air by a man driving an unlicensed car. 
 
 Violence in the region began on Sunday, when militants carried out the deadliest and most brazen attack against Egyptian troops in the Sinai region for decades. 
 
 There were further attacks on checkpoints in al-Arish on Wednesday, which left a number of people wounded. 
 
 Egypt launched its military offensive hours later, launching missile strikes from helicopters. 
 
 According to military officials, 20 people were killed in the village of Touma, while the Sheikh Zuwaid area to the west was also hit. 
 
 Egypt's military presence in Sinai is limited and requires Israeli approval under the terms of the 1979 peace treaty which returned Sinai to Egyptian control. 
 
 Analysts say that the security situation in the area has deteriorated following the fall of Hosni Mubarak last year, and that Islamist extremists appear to have gained a foothold. 
 
 There are hundreds of illegal tunnels along the Egypt-Gaza border that are used for smuggling people and goods. 
 
 The militants who launched Sunday's attacks are believed to have used them as an escape route. ||||| Image caption The border crossing between Israel and Egypt has been shut indefinitely 
 
 Unidentified gunmen have attacked a checkpoint on the Egyptian-Israeli border, killing at least 15 Egyptian policemen, officials say. 
 
 Israel said the heavily armed attackers, in two vehicles, then tried to smash their way across the border. 
 
 One vehicle apparently blew up at the North Sinai crossing, while the other was destroyed by the Israeli air force. 
 
 It is not clear who carried out the raid. Egyptian state television blamed Islamist militants. 
 
 The country's state news agency quoted a top security official who said the gunmen were "jihadists" who had infiltrated from the Gaza Strip. 
 
 Israel's defence minister Ehud Barak said his country's security forces had "thwarted an attack that could have injured many". 
 
 "The militants' attack methods again raise the need for determined Egyptian action to enforce security and prevent terror in the Sinai," he said in a statement. 
 
 Egypt's President Mohammed Mursi held an emergency meeting with military and security officials after the attack. 
 
 He condemned what he called a "cowardly" attack and said Egpyt would retake control of the Sinai. 
 
 "The [security] forces will take full control of these regions," the president said in a televised address. 
 
 Analysts say the security situation in the area has deteriorated following the fall of Hosni Mubarak last year. 
 
 In an earlier statement, he urged the authorities to take measures to "confront this serious challenge to Egyptian sovereignty and to protect Sinai from all armed groups". 
 
 At least seven other Egyptian policemen were wounded in the attack, security officials said. It is unclear how many militants were involved or how many were killed. 
 
 Security fears 
 
 Egyptian and Israeli forces say they are searching for any other gunmen who may have escaped. Israeli civilians in the area have been told to stay inside their homes. 
 
 The Rafah border crossing with the Gaza Strip has shut indefinitely, Egyptian state television says. 
 
 The attack on the border post close to Gaza and Israel took place around sunset, just as the guards had stopped work for the traditional iftar meal, which breaks the daily fast during the Islamic holy month of Ramadan. 
 
 Masked gunmen dressed as bedouin opened fire on police with guns and rocket-propelled grenades before driving off with an armoured vehicle, Egyptian state television said. 
 
 In Israel a military spokeswoman said two vehicles had been taken at its Kerem Shalom crossing. 
 
 The BBC's Yolande Knell in Cairo says the attack will be seen as more worrying evidence that Islamist militants have a firm foothold in the restive Northern Sinai region. 
 
 They have been blamed for several rocket attacks against Israel and a cross-border raid that killed nine Israelis last year. 
 
 Recently there have also been repeated attacks on the pipeline that exports gas to Israel and Jordan. Last month two Egyptian soldiers were shot dead. 
 
 Egypt's military sent extra tanks and troops into the Sinai last year. The terms had to be agreed with Israel under the terms of the 1979 peace treaty between the two countries, our correspondent says.
– Egypt's offensive in Sinai is growing. Yesterday saw missiles fired following a series of attacks on government forces; today, officials are sending troops and armored vehicles, including tanks, to the peninsula. Meanwhile, the military is closing off illegal tunnels across the border into Gaza; the 1,200 passageways, typically used by smugglers, are thought to have provided an escape for the militants who attacked soldiers on Sunday, the BBC reports. Local television earlier reported new fighting in Sinai, but a security insider later clarified the incident, saying a man driving an unlicensed car had been firing into the air.