The United Nations said it was withdrawing "all non-essential international staff" from Syria due to the worsening security situation.

It said up to 25 of about 100 foreign staff could leave this week.

More armoured vehicles were needed after attacks in recent weeks on humanitarian aid convoys and the hijacking of goods or vehicles, it added.

Some convoys had been caught in crossfire between Syrian government and rebel forces, including an incident in which two staff were injured near the airport.

"The UN has decided to send all non-essential international staff out of Syria and to halt all field trips outside of the capital for now," the UN Office for the Coordination of Humanitarian Affairs (OCHA) said in a statement.

Damascus had been considered safe until last week when the main airport was shut down and flights into Syria cancelled after several attacks by rebels, it said.

"The situation is significantly changing," said Sabir Mughal, the UN's chief security advisor in Syria. "There is an increased risk for humanitarians as a result of indiscriminate shooting or clashes between the parties."

Vladimir Putin in Turkey for Syria discussions

Russia and Turkey agreed to differ on strategies to end Syria's civil war, highlighting how distant the prospects of a negotiated solution to the 20-month conflict are.

Russian President Vladimir Putin held talks with Turkish Prime Minister Tayyip Erdogan during a one-day visit to Istanbul aimed partly at ensuring differences over Syria do not damage a deepening trade and energy relationship.

"The positions of the Russian Federation and Turkey completely correspond regarding what has to be attained (in Syria), but as of yet no shared approach regarding methods of how to attain it has been reached," Mr Putin told a joint news conference with Mr Erdogan after the talks.

Turkey - worried about Syria's chemical weapons, a growing refugee crisis, and Syrian support for Kurdish militants - has been a major backer of the Syrian opposition and has led calls for international action against President Bashar al-Assad.

It sees Russia, one of Syria's closest allies, as key to quelling a conflict that has sent over a hundred thousand refugees fleeing to Turkish soil and stirred warnings of a sectarian war beyond Syria's borders.

"Our biggest wish is an immediate halt to the bloodshed and fighting in Syria, and we are taking steps to make sure our foreign ministers are carrying out extensive work with this aim," Mr Erdogan said.

Turkish officials say Russia must be assured it does not stand to lose from the departure of Mr Assad, who has been Moscow's chief Middle Eastern ally.

Syria has been a major client for Russian arms and hosts a naval maintenance facility that is Russia's only military base outside the former Soviet Union.

Both sides have been careful to ensure their differences over Syria do not undermine a broader relationship governed by trade, Turkey's need for energy supplies and mutual security interests across an array of regional hotspots.

Patriot missiles

US Secretary of State Hillary Clinton is hopeful NATO allies will agree this week on stationing Patriot missiles in Turkey to defend against possible Syrian attacks, according to senior US officials.

Mrs Clinton also reiterated a warning against any attempt by the Syrian government to use its chemical weapons stockpile against the rebels, calling it a "red line" that would prompt US action.

The 28 NATO allies will meet in Brussels this week.

Turkey, which has formally asked NATO to help it bolster its air defences, is a big supporter of rebels fighting in a 20-month-old uprising to oust Mr Assad.