GAZA — Despite an Israeli concession to permit Gazans to fish up to six nautical miles from shore rather than three, Israeli forces detained a Palestinian fisherman and seized two boats as their crews tried to venture farther into the Mediterranean Sea.

Hamas, the militant faction that controls Gaza, denounced the action as a threat to the week-old cease-fire agreement that ended eight days of cross-border violence.

Israel has not officially announced any change in its naval blockade against Gaza, but a senior government official and a military official confirmed on Wednesday what they described as a “new arrangement” in which fishing boats would be allowed to venture twice as far from the Gaza coast as the previously enforced limit. The Oslo peace accords signed by Israeli and Palestinian negotiators in September 1993 called for allowing fishing up to 20 nautical miles from shore, but experts say that never came to fruition.

As indirect negotiations continued in Cairo between Israel and Hamas over issues like fishing, border crossings and movement in the buffer zone along the land boundary of the Gaza Strip, both sides said Wednesday that the encounter at sea and several other skirmishes in recent days had been problematic. A Hamas health official said seven Palestinians had been wounded by Israeli gunfire, one of them seriously, in the buffer zone on Wednesday. On Friday, a man was killed in the area as a crowd approached the fence between Gaza and Israel.

The senior Israeli official said the episodes were the Gazans’ “attempt to push the envelope on the new arrangement.” He added, “When we think there’s a violation, we respond.”

A spokeswoman for the Israel Defense Forces said the naval ships that patrol off Gaza had warned two boats of Palestinian fishermen to stop as they approached the six-mile mark before noon on Wednesday. When they did not stop, the Israeli sailors fired on the boats.

Nizar Ayyash, head of the Gaza fishermen’s syndicate, said that eight men had been questioned aboard an Israeli vessel, and that one of them had been taken to the port of Ashdod in Israel and held there all day. Mr. Ayyash, who contended that fishing stocks were insufficient close to the coast, said the Israelis had sunk one of the Palestinian boats, a claim the military spokeswoman denied.

Islam Shahwan, a spokesman for Hamas, which won elections in 2006 and took full control of Gaza in 2007, called the detention “maritime piracy.”

“We are in urgent need for the international community to force the enemy to respect the lull,” Mr. Shahwan said, referring to the cease-fire.

“By these unjustifiable actions,” he said, “the occupation threatens to destroy the lull.”

The cease-fire, announced Nov. 21 in Cairo, did not include specific changes to the blockade or rules governing the borders; instead, it called for discussions on those matters after the fighting stopped.

“The process of easing restrictions is exactly that: it’s a process,” the senior Israeli official said Wednesday. “If the quiet continues, it allows Israel to move forward in that process.”