Arab officials will prepare plans for sanctions against Syria on Saturday over its failure to let Arab League monitors oversee an initiative aimed at ending a violent crackdown on protests against President Bashar al-Assad.

Damascus missed a Friday deadline to sign an agreement under which the Arab League planned to send observers to Syria, where the United Nations says 3,500 people have been killed since the start of the uprising in March.

Despite Syria's pledge this month to withdraw its army from urban areas and let in the monitors, the violence has continued, prompting reprisals from the Arab League, stinging rebukes from Turkey and French proposals for humanitarian intervention.

Damascus says regional powers have helped incite the violence, which it blames on armed groups targeting civilians and its security forces.

Activists said government forces shot dead at least four demonstrators in Damascus on Friday who were appealing for foreign intervention to stop the crackdown. Two other civilians were killed in raids on their homes, they said.

The Syrian military said 10 personnel, including six pilots, were killed in an attack on an air force base on Thursday and that the incident proved foreign involvement in the eight-month revolt against Assad.

Arab ministers had warned that unless Syria agreed to let the monitors in, they could consider imposing sanctions including suspending flights to Syria, stopping dealings with the central bank, freezing Syrian government bank accounts and halting financial dealings.

They could also decide to stop commercial trade with the Syrian government 'with the exception of strategic commodities so as not to impact the Syrian people,' the ministers said.

The League's economic and social council, which can comprise officials or ministers, will meet on Saturday to prepare recommendations for a meeting of foreign ministers the next day.

Syria's economy is already reeling from months of unrest, aggravated by U.S. and European sanctions on oil exports and several state businesses.

In neighboring Turkey, Foreign Minister Ahmet Davutoglu said his country could take steps alongside the Arab League if Syria did not respond to the proposal for observers positively.

'I want to say clearly we have no more tolerance for the bloodshed in Syria,' he said.

The stepped-up pressure followed a French proposal for 'humanitarian corridors' to be set up through which food and medicine could be shipped to alleviate civilian suffering.

The French plan could link Syrian civilian centers to the frontiers of Turkey and Lebanon, to the Mediterranean coast or to an airport, and enable supply of humanitarian supplies or medicines to people in need.

French Foreign Minister Alain Juppe said the plan fell short of a military intervention but acknowledged that humanitarian convoys might need armed protection.

Foreign powers are seeking to persuade Damascus to accept such a scheme, diplomats say.

'So far they haven't said no, so we may be able to convince them,' said a Western diplomatic source. 'As long as we are in the humanitarian dimension it is harder for countries like Syria to refuse to allow aid to civilians.'

Without Syrian agreement, the only way humanitarian corridors could work is if they were backed by force, ideally supported by a UN resolution.

Some a measure of comfort for Assad came from longtime ally Russia, China and other countries, which expressed opposition to sanctions and warned against a foreign military intervention.

'At the current stage, what is needed is not resolutions, not sanctions, not pressure, but internal Syrian dialogue,' Russian Foreign Ministry spokesman Alexander Lukashevich said.

Lukashevich said Russia supported the Arab League's call for a halt to the violence but that 'radical opposition' groups with foreign support shared the blame. Outside military intervention was 'absolutely unacceptable.'

After a meeting in Moscow on Thursday, diplomats from Russia, China and the other three emerging-market BRICS countries - Brazil, India and South Africa - also warned against foreign intervention without U.N. backing.

Alongside the mainly peaceful protests, armed insurgents have increasingly attacked military targets in recent weeks. Officials say 1,100 members of the security forces have been killed since the outbreak of uprising.

A military spokesman said on Friday an 'armed terrorist group' killed six pilots and four other personnel on an air force base between Homs and Palmyra, the day before.

'This confirms the involvement of foreign elements and their support of these terrorist operations in an effort to weaken the fighting capabilities of our forces,' he said.

The account fits the government narrative that it faces an armed insurrection by trouble-makers backed by its enemies, rather than a largely peaceful pro-democracy movement inspired by the Arab Spring revolts which toppled the rulers of Tunisia, Egypt and Libya and may have forced out Yemen's president.

State television also showed pictures of thousands of people demonstrating in central Damascus 'expressing their rejection of the Arab League decision against Syria.'-Reuters