Two rockets were fired into southern Israel by Palestinian militants in Gaza on Friday, causing no casualties but triggering a series of Israeli air raids.

No one was hurt in either of the attacks. One of the rockets hit a parked bus in the town of Sderot, while the Israeli army said the second was destroyed by its "Iron Dome" defense system over the town of Ashkelon.

The Gaza Strip, ravaged by wars and nearly a decade of a grueling Israeli blockade, could become uninhabitable for residents within just five years, the United Nations development agency said Tuesday.

"The social, health and security-related ramifications of the high population density and overcrowding are among the factors that may render Gaza unlivable by 2020," the U.N. Conference on Trade and Development (UNCTAD) wrote in its annual report.

Some 35 aid groups from around the world, including ActionAid and Oxfam, launched a joint call Wednesday to end Israel's blockade of the Gaza Strip, which is in dire need of reconstruction.

Last summer's devastating war in Gaza caused mass casualties, killing 2,251 Palestinians, including more than 500 children. Seventy-three people were killed on the Israeli side, including 67 soldiers.

A strike by teachers and personnel in Gaza kept more than 200,000 children from returning to school for the new term Monday, as the U.N. agency that employs them struggles financially.

Several thousand teachers, assistants and administrative personnel protested in front of the headquarters of UNRWA, the United Nations agency for Palestinian refugees.

Almost 80,000 people had by Monday signed a petition urging the arrest of Israeli Prime Minister Benjamin Netanyahu for war crimes when he visits London next month.

The petition was launched earlier this month by British citizen Damian Moran and is posted on the government's website.

The exiled head of Palestinian Islamist movement Hamas, Khaled Meshaal, said "positive contacts" have been made towards securing a long-term truce with Israel, in a video posted Friday on an Arab website.

Meshaal's comments posted by pan-Arab news website al-Arabi al-Jadid were the first by a leader of Hamas, which does not recognize Israel, to openly confirm such contacts with the Israelis.

Gunmen in Egypt boarded a bus carrying Palestinians from Cairo to Gaza and seized four of the passengers before letting the others continue, Hamas officials in the strip said early Thursday.

Interior ministry spokesman Iyad Bezem said in a statement that the kidnapping took place late Wednesday night in the troubled northern Sinai region as the bus headed for the Rafah border crossing into Gaza.

An official of the Palestinian Islamist movement Hamas has been detained at Cairo airport as he tried to enter Egypt, an airport source said Tuesday.

Hassan al-Saifi, whose name figures on an Egyptian watch list, was detained on arrival Monday, the source said, adding he was still at the airport and would likely be deported.

The Palestinian Authority on Tuesday accused Hamas of holding secret talks with Israel that would endorse the separation of Palestinian territories.

Arab and Turkish media have carried reports, picked up by Israeli media, claiming Israel and the Palestinian Islamist movement that rules the Gaza Strip were holding talks.

The infant mortality rate in Gaza has risen for the first time in more than half a century, a new study by the U.N. aid agency for Palestinian refugees says.

"The number of babies dying before the age of one has consistently gone down over the last decades in Gaza, from 127 per 1,000 live births in 1960 to 20.2 in 2008. At the last count, in 2013, it had risen to 22.4 per 1,000 live births," a statement from the UNRWA relief agency released at the weekend said.