A Turkish policeman was shot dead and another was wounded after unknown assailants opened fire on the officers in Istanbul, the city's police chief said Saturday.

The attack occurred in the populous neighborhood of Gaziosmanpasa, located on the European side of Istanbul, police chief Huseyin Capkin told the Anatolia news agency.

Syrian opposition leaders met Saturday for a second day of talks with hopes of inking the founding principles of a future Syria, now tarnished by a 16-month long bloodshed and political chaos.

Major Syrian opposition groups gathered once more in Istanbul under the umbrella of the main opposition, the Syrian National Council (SNC), recognized by most factions as the official voice, and a "legitimate representative of Syrians" by Western and Arab powers.

Syrian opposition leaders were meeting in Turkey on Friday in a bid to settle their differences and forge a united front to confront the escalating conflict in their homeland.

"We will work towards a unified vision," said Burhan Ghalioun, who stepped down last month as head of the main opposition group the Syrian National Council in the face of mounting splits that were undermining its credibility.

A Turkish prosecutor has linked a shadowy group the government suspects of being coup plotters to the 2007 murders of three Christian missionaries, media reports said Saturday.

The three members of a Bible publishing firm, including a German national, were tortured and killed in April 2007 in the eastern town of Malatya. Nine people are already on trial for the murder, of whom six are in jail.

U.S. Secretary of State Hillary Clinton called on Iran Thursday to be ready to take "concrete steps" on its disputed nuclear program ahead of a Moscow meeting between Tehran and world powers.

Iran and the so-called P5+1 group -- the United States, Britain, France, China, Russia and Germany -- meet on June 18-19 in the Russian capital to discuss Iran's nuclear program which the West fears aims to build the bomb.

Al-Qaida is still a serious and imminent threat as it is spreading geographically, U.S. Secretary of State Hillary Clinton said Thursday.

"The core of al-Qaida that carried out the 9/11 attacks may be on the path to defeat, but the threat has spread, becoming more geographically diverse," Clinton said ahead of a counterterrorism meeting.

U.S. Secretary of State Hillary Clinton will meet her British, French, Turkish and some Arab counterparts Wednesday to "discuss the situation in Syria," a Turkish diplomatic source said.

Britain's foreign minister, William Hague, France's Laurent Fabius, Turkey's Ahmet Davutoglu and several Arab ministers will be at the informal evening meeting in Istanbul on the 15-month-old crisis, the source said.

Ministers from some 30 countries, including U.S. Secretary of State Hillary Clinton, will meet in Turkey on Thursday at an anti-terror forum overshadowed by Syria's crisis and Iran's nuclear threat.

The meeting comes as Western powers are pushing for increased pressure on Syrian President Bashar al-Assad to stop his regime's assault against the population.

U.N. chief Ban Ki-moon warned Thursday of a "catastrophic civil war" in Syria after the massacre of more than 100 civilians in the town of Houla that sparked global horror and outrage.

"The massacres of the sort seen last weekend could plunge Syria into a catastrophic civil war, a civil war from which the country would never recover," he told an Istanbul forum of the U.N.-led Alliance of Civilizations initiative.

Somalia needs a global reconstruction effort to back up ongoing stabilization efforts and stop the Horn of Africa's 20-year descent into chaos, leaders said Thursday at the start of a meeting in Turkey.

Representatives from 54 countries gathered in Istanbul to find a path towards a better future for a country that was the reason the term "failed state" was coined two decades ago.