Turkey cannot stand by and watch if Syria's crackdown on a popular uprising puts security in the region at risk, Foreign Minister Ahmet Davutoglu said on Friday.

While Ankara has no desire to interfere in Syria's internal affairs, it has a duty to tell Damascus Enough! if it threatens Turkey's security by fighting its own people and forcing people to flee the country, he said.

Turkey, which has the second-largest army in NATO, has steadily ratcheted up its warnings to Syrian President Bashar al-Assad to halt the violent suppression of protests in which more than 4,000 people have been killed, according to the United Nations.

It said last month it did not want any military intervention in Syria but that it was ready for any scenario including setting up a buffer zone inside its southern neighbour.

Turkey, which has become increasingly critical of its one-time ally, fears an all-out civil war along sectarian lines in Syria could spill across its borders and spark tensions among its own people.

Turkey has no desire to interfere in anyone's internal affairs but if a risk to regional security arises then we do not have the luxury of standing by and looking on, Davutoglu told reporters.

If a government that is fighting its own people and creating refugees, is putting not only their own security at risk but also that of Turkey, then we have a responsibility and the authority to say to them: 'Enough!' he said.

France, which has led Western criticism of Assad, has also ruled out military intervention, but called last month for a secured zone to protect civilians in Syria. Davutoglu has said a militarised buffer zone was not on today's agenda.

NOTHING TO HIDE

Muslim Turkey was once one of Syria's closest regional friends, and Turkish Prime Minister Tayyip Erdogan had built a strong rapport with Assad.

But as the violence grew worse and Assad ignored Ankara's advice to halt the crackdown on protesters and make urgent reforms, relations broke down. Erdogan has now bluntly told Assad he should quit.

Turkey now gives refuge to Syrian army defectors and the Syrian National Council, an umbrella organisation for exiled Syrian opposition figures.

Last week, it followed the Arab League by announcing a list of economic sanctions on Syria it said would target the government, including freezing state assets and imposing a travel ban on officials and suspending financial transactions.

On Friday, Davutoglu called on Assad to accept an Arab League demand to send in independent observers.

This is our argument together with the Arab League: if you do not want outside intervention, let a committee from the Arab League come in and observe, he said.

If there are terrorist actions, if there are rebellious actions then let them report those, but if there is a peaceful demonstration let them also witness that. If he has nothing to hide then he must say 'yes' to this.

Syria has suspended a free trade agreement between the two countries, as well as imposing a 30 percent tariff on all Turkish imports and prohibitive duties on fuel and freight.

Turkey responded this week by imposing a similar levy on all Syrian goods arriving in the country and has also started exporting goods to the Middle East via sea or through Iraq, bypassing traditional routes in Syria.

(Reporting by Tulay Karadeniz; Writing by Jonathon Burch; Editing by Jon Hemming and Mark Trevelyan)