More than 1.5 million Syrians have fled their conflict-ravaged homeland, the U.N.'s refugee agency said Friday, warning that the real figure could be even higher as the tally only reflected those who register with aid groups.

Dan McNorton, spokesman for the U.N. High Commissioner for Refugees, told reporters that close to 250,000 Syrians were being registered each month.

Turkey has arrested one of the major suspects in the twin car bombings that killed at least 51 people in a city near the Syrian border, the city's governor said on Friday.

"We captured one of the chief actors involved in the incident at 11:40 pm last night," said Celalettin Lekesiz, governor of southeastern Turkey's Hatay city where the bombs went off.

Moody's has raised Turkey's bond rating to investment grade, the rating agency said Thursday, citing improved economic and public finance measures and progress on reforms.

The government-bond ratings went up a notch to Baa3, the first step into investment grade, from Ba1, with outlook considered stable.

U.S. President Barack Obama and Turkish Prime Minister Recep Tayyip Erdogan vowed to crank up pressure on Syria's President Bashar Assad Thursday, but offered no concrete new measures to do so.

Obama warned there was no "magic formula" to force Assad to leave power, as both the United States and Turkey want, but said he hoped a conference that Washington is organizing with Russia next month would be successful.

A Turkish parliamentary committee is set to debate Wednesday a proposal by the Islamic-rooted government to introduce new curbs on the consumption and advertising of alcohol, a parliamentary source said.

The supporters of the measure say the ruling Justice and Development Party (AKP) bill seeks to protect society, particularly children, from the harmful effects of alcohol.

Taliban insurgents on Tuesday announced they had released the final four Turkish engineers held hostage since their helicopter crash-landed in eastern Afghanistan last month.

"The remaining four Turkish nationals were released today as a sign of goodwill and as an Islamic and humanitarian gesture of respect to the Muslim Turkish nation," the insurgents said in an emailed statement.

Turkish Prime Minister Recep Tayyip Erdogan on Tuesday firmly rejected a Syrian proposal to hold a joint probe over a twin car bomb attack that killed 51 people.

"The administration in power in Damascus is illegal ... how can we recognize a structure that isn't even recognized by its own citizens," Erdogan told reporters before leaving for a meeting with U.S. President Barack Obama in Washington on Thursday.

Five Arab countries and Turkey have reiterated that President Bashar Assad should have no role in the future of Syria, as Russia and the United States proposed a peace conference.

The foreign ministers of Egypt, Jordan, Qatar, Saudi Arabia and the UAE, as well as that of Turkey -- all supporters of the Syrian opposition -- expressed their stand at a meeting late Monday in Abu Dhabi, WAM Emirati news agency said.

The Iraqi government said on Tuesday that it rejects the entry of Kurdish fighters from Turkey into its territory as a "flagrant violation" of its sovereignty that damages relations with Ankara.

"The Iraqi government confirms its rejection of the withdrawal and the presence of armed men of the Kurdistan Workers' Party (PKK) inside Iraqi territory, which is a flagrant violation of Iraq's sovereignty and independence," a statement from the cabinet said.

A Turkish F-16 fighter jet crashed in south-eastern Turkey on Monday near the Syrian border, and its pilot is believed to have safely ejected, an army statement said.

"Radio communication was cut around 2:15 pm (1115 GMT) with the F-16 fighter plane on mission in the Amanos mountains," shortly after the pilot signaled he was about to eject from the aircraft, read the statement.