GAZA, May 29 (Xinhua) -- Israel and the Gaza Strip witnessed a new round of escalating tensions on Tuesday, as Israel launched a massive airstrike in response to rockets' firing from Gaza and intercepted a Palestinian boat attempting to break Israel's maritime blockade.

On Tuesday afternoon, Israeli fighter jets carried out a series of airstrikes on a cross-border tunnel and over 35 targets in Gaza, most of which belong to the Islamic Hamas movement, which rules the enclave, and the Palestinian Islamic Jihad, an Islamist group in Gaza.

Palestinian security sources in Gaza told Xinhua that the targets were mostly training posts and military facilities, and the Israeli attack was in response to the firing of 30 mortar shells from Gaza into southern Israel on the morning.

The sources said the Israeli airstrike also targeted a site of the Hamas-run police and destroyed a tunnel which was under construction between southern Gaza and Israel.

The Education Ministry in Gaza confirmed that a school was also damaged due to the Israeli airstrike in central Gaza while students were having final exams. No injuries were reported.

It said in a statement that "Israel has to be punished for its crimes against the Palestinian people."

No one has claimed responsibility for the mortar shells fired from Gaza into Israel in the morning, however, Israel held the Islamic Jihad responsible.

Spokesmen of both Hamas and the Islamic Jihad hailed the mortar attacks on Israel, justifying them as "a natural response to the Israeli crimes in Gaza."

The Islamic Jihad "blesses the response today of the armed resistance which defends our people in the face of the terrorist occupation," said Dawood Shihab, spokesperson of the Islamic Jihad.

However, the group does not intend to go into war with Israel, and only responds to Israel's blockade and occupation of Palestinian territory, Shihab said.

Fawzi Barhoum, Hamas' spokesman in Gaza, said in an emailed statement that "what the armed Palestinian resistance has done today morning came as part of the natural right of defending our people."

"The attacks were a response to the crimes that Israel has committed towards our fighters in Rafah and in northern Gaza Strip in the past two days," said Barhoum, adding that Israel is responsible for any upcoming escalation.

The international community has called for restraint by both sides to avoid further escalation of violence.

An Islamic Jihad official, who spoke on condition of anonymity, told Xinhua that Egypt contacted his group in the afternoon in an effort to prevent further deterioration of the situation between Gaza and Israel.

For his part, Nickolay Mladenov, United Nations special coordinator for the Middle East peace process, called on the Palestinian factions and Israel to exercise restraint and avoid escalation between the two sides.

"Palestinians and Israelis must act to prevent life-threatening incidents," Mladenov said in a press release sent to Xinhua.

Mladenov expressed his deep concern over the indiscriminate firing of rockets from Gaza towards communities in southern Israel, saying that "the attacks are unacceptable and undermine the serious efforts of the international community to improve the living situation in Gaza."

Also on Tuesday, Israeli naval gunships forcibly intercepted a Palestinian boat carrying 20 passengers on board in an attempt to break the 11-year maritime blockade imposed by Israel on the enclave, according to the Palestinian National Committee, organizer of the voyage.

The passengers included patients who seek medical treatment abroad, college students studying in foreign universities, and people attempting to seek jobs overseas.

The vessel planned to sail out of the blockade zone of 6 nautical miles (11 km), but was stopped by four Israeli gunships after travelling for 12 nautical miles, said the committee in a statement.

It added that it lost contact with the boat 30 minutes after it was seized, calling on the international community to protect the Palestinians on board.

Meanwhile, an Israeli military spokesperson said in a statement that the boat was apprehended after it "attempted to breach the legal naval blockade."

"After a search of the boat and the Palestinians, the boat will be towed to the Israeli navy base in Ashdod," the spokesperson said.

Israel accused Hamas of standing behind the sailing and "trying to carry out propaganda operation whilst cynically using its people."

Several international flotillas have tried to break the blockade but were stopped by the Israeli navy. In 2010, six ships, carrying humanitarian aid and construction materials to Gaza, were blocked by Israel's navy commando unit in international waters off Gaza. Ten Turkish activists were killed in the raid.

Israel has imposed a blockade on Gaza since 2007, shortly after Hamas took over the enclave.

Under Israel's restrictive policy, Palestinians are not allowed to leave or enter the Gaza Strip. Israel allows access of only a few goods to the enclave and most imports are banned too.

With the collapse of the infrastructure, Gaza residents have only four hours of electricity per day, and 97 percent of the drinking water is polluted, according to official Palestinian figures.