Fierce clashes broke out on Saturday in Cairo between protesters angry at the ruling military's handling of the transition from Hosni Mubarak's regime and army loyalists, an Agence France Presse correspondent said.

Around 1,000 protesters had tried to reach the defense ministry, the headquarters of the Supreme Council of the Armed Forces-- which took power when Mubarak was ousted-- but were blocked by barbed wire and armored personnel carriers.

Iranian nuclear scientist Dariush Rezaei was shot dead on Saturday when unknown assailants riding a motorcycle targeted him in the capital Tehran, local media reported.

"A physics professor and nuclear scientist was assassinated a few hours ago in front of his house in Tehran," Mehr news agency reported at 14:30 GMT.

Four Yemeni suspects have been arrested in connection with a car-bomb attack in the southern port of Aden that killed a British businessman on Wednesday, security officials said on Saturday.

The four suspects lived in the Moalla neighborhood where the attack took place, the security officials said, without ruling out further arrests.

Egypt's military ruler on Saturday stressed the army's commitment to democracy, as protesters kept up pressure on the general over the slow pace of reforms since a revolt ousted Hosni Mubarak.

Field Marshall Hussein Tantawi, the head of the Supreme Council of the Armed Forces and Mubarak's longtime defense minister, pledged to work for a free system through fair elections and a constitution.

Syrian security forces on Saturday arrested several civilians, including women, in the flashpoint central city of Homs where gunfire was also heard, activists told Agence France Presse.

"Gunshots were heard in al-Khalidiyeh neighborhood and security forces have been making arrests," Abdel Karim Rihawi, the head of the Syrian League for the Defense of Human Rights, said by telephone.

South Sudan rebel leader Gatluak Gai was shot dead in Unity state on Saturday, the army and a rebel source said, just a week after agreeing to a ceasefire and as the newly independent country renewed its amnesty offer.

It was unclear how Gai was killed, with the SPLA, the army of the south, insisting he was shot by his own men after reneging on the agreement, and a rebel source saying he was "murdered" by the SPLA.

Egypt's military rulers on Saturday accused the April 6 pro-democracy group, which helped launch the uprising that ousted Hosni Mubarak, of sowing strife after hundreds tried to march to the defense ministry.

The Supreme Council of the Armed Forces -- which took power when former president Mubarak was toppled in February -- accused April 6 of "driving a wedge between the people and the army."

Turkish Prime Minister Recep Tayyip Erdogan said on Saturday it was "unthinkable" for Ankara to normalize relations with Israel until it apologized over its bloody raid on a Turkish ship heading for Gaza.

"As long as Israel does not apologies to Turkey, pay compensation to the families of the victims and lift its blockage on the Gaza strip, a normalization of relations is unthinkable," he said at the opening of a meeting of Palestinian ambassadors in Istanbul in the presence of President Mahmoud Abbas.

Palestinian President Mahmoud Abbas said Saturday the Palestinians' bid to win U.N. backing for statehood was forced upon them by Israel's refusal to halt settlement building and end its occupation.

"We are going to the United Nations because we are forced to, it is not a unilateral action," he said in a speech in Arabic in Istanbul, where he is meeting Palestinian diplomats from around the world.

A passenger train has derailed and caught fire in central Syria, killing the driver and wounding several passengers, after "saboteurs" tore out part of the tracks, the state-run news agency, SANA, said.

The train with 480 passengers on board was traveling Saturday from the northeastern city of Aleppo to the capital Damascus.