Thaer al-Bik, 30, and Muhammad al-Khalidi, 26, on Thursday went to inspect a tunnel in northern Gaza that was damaged Tuesday by an Israeli airstrike, the brigades said in a statement.
Members al-Bik and al-Khalidi inhaled an unidentified gas in the tunnel and died from suffocation, the group said.
On Sunday evening, Israel bombed a site used by the Al-Qassam Brigades, the military wing of Hamas, south of Gaza City, Palestinian medics said. Israel's army said it hit "a terror activity site" in the area.
Over the next three days, eight Palestinians -- including two Al-Qassam Brigades fighters -- were killed in a series of Israeli airstrikes and tank shelling.
Militants in Gaza fired a volley of rockets across the border. The Al-Qassam Brigades joined the confrontation on Tuesday, launching its first rockets in over a year.
It claims to have fired 120 projectiles at Israeli military sites around Gaza before agreeing to an Egypt-brokered truce on Wednesday night.
Hamas leader Ayman Taha told Ma'an that the party wanted a truce to avoid Israeli attacks against Palestinians but added "aggression cannot pass without punishment."
"The Al-Qassam Brigades did not put its weapons down and it is there in the field and will respond to any Israeli aggression at the appropriate place and time," Taha said.
Hamas usually holds its fire under an unofficial truce with Israel, and discourages smaller militant groups from firing at Israel.
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