GAZA CITY, Gaza Strip — 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.

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.

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.

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

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 told The Associated Press.

“We demand that Egypt explain its position about what is happening and hold those responsible accountable,” he said.

The United States and Israel have been pushing Egypt to do more to close the tunnels, which provide Hamas with a lifeline.