Egyptian forces pumped gas into a cross-border tunnel used to smuggle goods into the Gaza Strip on Wednesday, killing four Palestinians, Hamas officials said.

Egypt has been under pressure to seal off the hundreds of tunnels that are a key economic lifeline for the blockaded Palestinian territory but which are also used to bring in weapons for the Islamic militant group.

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Israel and Egypt have kept Gaza's official border crossings closed since Hamas seized control of the coastal strip in 2007 from forces loyal to Western-backed Palestinian President Mahmoud Abbas, who now only governs in the West Bank.

A Hamas security official in charge of the tunnel area along the border said the Egyptians filled the passage with some type of crowd dispersal gas. He spoke on condition of anonymity because he was not authorized to give his name.

The Hamas Interior Ministry later said in a statement the gas used to try to clear the tunnel was poisonous. Besides those killed, six people were injured, it said.

"The Interior Ministry confirms that the citizens' cause of death was the Egyptian security forces spraying poison gasses into one of the tunnels," the statement said without elaborating.

A doctor at a hospital in the Gaza border town of Rafah, Hamdan Abu Latifa, said the dead smugglers suffocated.

An Egyptian border security official refused to comment Wednesday.

'Hold those responsible accountable'

Egyptian security forces have sprayed gas into tunnels before, according to Hamas officials and tunnel operators. But Wednesday's incident was the first time people have died as a result, they said.

A Hamas official demanded an explanation.

"This is a terrible crime committed by Egyptian security against simple Palestinian workers who were trying to earn their daily bread," Hamas spokesman Fawzi Barhoum to The Associated Press. "It was a killing in cold blood. Hamas and all the Palestinian people condemn it strongly."

"We demand that Egypt explain its position about what is happening and investigate the circumstances of this terrible crime and show the truth to the entire world and hold those responsible accountable," he said.

The United States and Israel have been pushing Egypt to do more to try to close the tunnels, which provide Hamas with a lifeline helping it to stay in power in Gaza. Weapons and other contraband regularly move through the tunnels.

But the 1.5 million residents of the impoverished Gaza Strip also rely on the tunnels to bring in food and commercial goods like refrigerators and clothing.

Israeli aircraft have targeted the tunnels in bombing runs in the past, particularly during its military offensive against Gaza more than a year ago to stop daily rocket attacks. Tunnel workers have also been killed when passageways accidentally collapsed.

Many of the tunnels, dug with electrical drills and running side by side under the border, are just high enough to enable workers to move on all fours. Their entrances are covered by tents and they are equipped with motorized pulleys to haul goods and generator-powered lighting.

Palestinian officials in Rafah say Egypt has stepped up a crackdown on smuggling in recent months, blowing up numerous tunnel entrances on its side of the border, setting up checkpoints in the area and confiscating contraband. Since December, Egypt has also been building an underground steel wall to block the tunnels.

Rafah officials have said about four miles (six kilometers) of that barrier — covering roughly half of the border area — is already complete.

The officials say the Egyptian measures have led to a sharp slowdown in tunnel traffic in recent months, pinching the local economy.