The Israelis say weapons and know-how supplied by Lebanese ally Hezbollah make Hamas a more formidable foe.
Four days after Israel launched a withering ground assault on Hamas fighters in their stronghold of Shejaia following intensive air strikes, the army still misses the control of the area. Smoke from shelled homes and the buzz of Israeli drones clog the sky above the wrecked district on Gaza's eastern border.
Exploiting a vast network of secret tunnels to snipe at enemy troops and blast their vehicles even inside Israel, Hamas has killed 32 Israeli soldiers -- almost three times as many as in the last major ground clashes in a 2008-9 conflict.
"The al-Qassam brigades continue to give repeated surprises, and every day the holy warriors arise from where the (Israeli) occupation could not foresee," the group said last week.
"They fight... face to face with the enemy in retaliation for the blood of the martyrs that the occupier spills daily."
The action has lived up to the fierce rhetoric. Hamas has far outstripped fellow fighters in Islamic Jihad and other groups in sending drones, scuba commandos and tunnel raiders to take the fight into Israel.
In one such infiltration, Hamas fighters emerged wearing full Israeli uniform, but were let down by one key detail – they were carrying Kalashnikov rifles, not standard issue M16s or Tavor assault rifles.
In the most deadly incident for Israel yet, on the first day of its incursion to begin destroying the tunnels on Sunday, Hamas says its fighters watched as an enemy armored personnel carrier lurched into a web of booby traps they had laid.
"Our holy warriors detonated the minefield with such force that (the carrier) was destroyed. They advanced on it, opened its doors and finished off all left inside," the group said.
The Israeli military acknowledges Hamas' increased skill.
"They have undergone extensive training, they are well supplied, well motivated and disciplined. We have met a more formidable enemy on the battlefield," said Israeli military spokesman Lieutenant-Colonel Peter Lerner.
“What is remarkable is that in the past eight years they have basically built an underground Gaza. It's astonishing," he said.
Confined in the crowded sandy coast enclave of 1.8 million, where poverty and unemployment hover around 40 percent, weary Gazans say they hope the battle will break the blockade that Israel and Egypt impose on them. They have very little to lose.
Hamas leaders hope to achieve that goal through the prowess of their men at the front, trained to inflict casualties and grab soldiers to gain political leverage.
There are conflicting reports about Hamas's losses in terms of fighters. Since Israel has claimed Hamas's armed wing numbers 20,000 men, the martyrdom of 60 or 70 can not be described as a huge loss.
In 22-days of fighting during a 2008-9 conflict, Hamas and other militant groups largely melted away, allowing Israeli tanks to approach the outskirts of Gaza City. The fighters killed just six soldiers then, and two more in an eight-day round of battle in 2012, which did not escalate into an Israeli ground incursion.
Israel's loss of 12 soldiers on Sunday was the largest single-day toll since its forces pushed into South Lebanon in an attempt to dislodge Hezbollah militants there in 2006.
Reuters