The tunnels provide a lifeline for those living in the impoverished Gaza Strip
Four Palestinians have died in a smuggling tunnel under Egypt's border with the Gaza Strip, Palestinian medical officials say.
The men died because of an explosion near the Egyptian side, the head of emergency services in Gaza said.
Some Gazans accuse Egypt of using dynamite and pumping gas into the tunnels to end Palestinian attempts to beat the Israeli-Egyptian blockade.
However, flammable items such as petrol are frequently transported underground.
Hamas, the Islamist movement that controls Gaza, accused Egypt of "spraying poison gas" into the tunnel.
Egyptian security officials have denied the allegation they used poison gas.
An unnamed security source told the Associated Press that Egypt routinely blows up the mouths to the tunnels to seal them off, and that the blast and an ensuing fire could quickly use up all the oxygen in the confined space causing people caught inside to suffocate.
French news agency AFP quoted unnamed officials as saying Egyptian security forces had destroyed four tunnels but were unaware of any casualties.
Egypt is building a huge underground barrier along the Gaza border to stop smuggling.
The structure - made of bomb-proof steel - will be 10-11km (6-7 miles) long and extend 18m (59ft) below the surface.
Under the blockade, Israel allows only limited humanitarian goods into the strip, saying it wants to pressure Hamas and stop it smuggling in weapons, including the some of the rockets that Gaza militants fire into southern Israel.
The tunnels are used to smuggle in arms, fuel and goods from Egypt, but cave-ins are frequent.
Egypt keeps its pedestrian border crossing with the Gaza Strip closed most of the time.
In a separate incident, a Palestinian man died after being shot during a protest near Gaza's border with Israel.
Palestinian medics said Ahmed Salim, 20, died after being shot by Israeli forces in the thigh.
The Israeli military said live rounds were fired as "warning shots" in response to a "riot", where about 50 Palestinians were throwing stones and starting fires.
It said the incident had taken place 50m from the border fence with Israel, but that it considers the area 300m from the fence to be a "combat zone".
Adie Mormech, an activist with the International Solidarity Movement group, said that although stones were being thrown, none reached the fence, and there was no warning fire before the shot that hit Mr Salim.
Palestinian and international demonstrators have increased protest activity in the buffer zone in recent months. Israel maintains a policy of firing at anyone present in the area.
Israel says the zone is necessary for security, as militants frequently approach the fence to try to plant explosives and attack Israeli forces.
But Palestinians complain that the buffer zone renders swathes of agricultural land unusable.