Israeli warplanes bombed two separate targets in the Gaza Strip Thursday after Palestinian militants fired projectiles across the border, the army said.

The strikes in northern Gaza came just hours after witnesses reported an Israeli army incursion across the Strip's eastern borders, to which Islamist militants claimed to have responded with mortar fire.

The military said "aircraft targeted two concealed rocket launchers in the northern Gaza Strip" in response to two projectiles that had hit Israel since Wednesday.

An spokeswoman told AFP one of the projectiles was a rocket and the other a mortar round.

"Launching rockets against Israel and its civilians is a breach of our sovereignty. We maintain the right to operate against those who are involved in terror," said army spokesman Lieutenant Colonel Peter Lerner.

No casualties were reported in any of the attacks from either side.

Witnesses said earlier that Israeli troops backed by armour entered Gaza east of the southern town of Khan Yunis.

"Six military bulldozers accompanied by several tanks entered about 200 metres (yards) from the border into farmland, as helicopters and spy planes circled above near the village of Khuzaa," one witness told AFP.

"Resistance fighters fired several mortar rounds at the Israeli forces and explosions were heard in the area."

Militant group Islamic Jihad said its fighters responded to the incursion with "four mortar shells," but made no mention of rocket fire, and did not specify where the shells had hit.

Rival Islamist group Hamas, which runs the territory's administration, detained two of the fighters and held them for hours before releasing them, an Islamic Jihad spokesman said.

Hamas has endeavoured to enforce a year-old Egyptian-brokered truce with Israel which ended the last major outbreak of fighting in and around Gaza late last year.

On Thursday, thousands of members of Hamas' armed wing, the Ezzedine al-Qassam Brigades, paraded in the Gaza Strip to mark the anniversary of that fighting, named Operation Pillar of Defence by the Israelis.

Across Gaza, hundreds of masked militants paraded in pick-up trucks, some with anti-aircraft guns and rocket launchers.

Senior Hamas official Mahmoud Zahar, accompanied by premier Ismail Haniya, gave a speech to thousands of fighters in which he talked of a "war of the tunnels".

"The war of the tunnels in the Gaza Strip started when we decided to strike before they struck us," he said, referring to the discovery of a tunnel from Gaza into Israel made public by Israeli authorities in October.

Meanwhile, Amnesty International criticised both Israel and Hamas for failing to investigate alleged rights abuses that took place during the 2012 fighting in Gaza.

"Israel's Military Advocate General has received scores of complaints from Palestinian and Israeli NGOs, including cases of civilians who were killed in attacks which may well have been war crimes, but has yet to open a single criminal investigation," Amnesty said.

The rights group also said Hamas "has not conducted investigations of any kind into violations of international humanitarian law by Palestinian armed groups during the conflict".

While Hamas has tried to enforce the Egyptian-brokered truce with Israel since 2012, four Hamas fighters were killed and five Israeli soldiers wounded in an exchange of fire on November 1.

The casualties came during an incursion by the Israeli army to destroy a tunnel under the border it alleged was intended as a springboard for attacks inside the Jewish state.

The exchanges of fire came as Middle East peace talks faltered, with the resignation of the entire Palestinian negotiating team in protest against continued Israeli settlement building.