President Abdullah Gul of Turkey says he does not want his country to intervene in the violence taking place in neighbouring Syria.
During a state visit to London, Mr Gul said Turkey would defend its interests.
He told the BBC that "fundamental reforms'' were needed in Syria.
Mr Gul said Turkey no longer trusted its former ally, President Bashar al-Assad.
Turkish Prime Minister Recep Tayyip Erdogan has said Syria's future cannot be built on "the blood of the oppressed".
Mr Gul said that the Syrian regime had now reached a "dead end" and that the country was prepared for the worst-cast scenario.
"(In) this age, authoritarian regimes or one-party rule are no longer acceptable, first by the people of that country, and of course, (by) the region and the world.''
Mr Gul said that despite frequent personal contact with Mr Assad, assurances that reform in the country were imminent had not been followed by concrete change.
"Unfortunately, it was too little, too late," he said. "So therefore we don't have any more trust."
Mr Gul said the Syrian people deserved the changes they sought, and had been forced on to the streets as their "legitimate demands" were not being met.
"We don't want to interfere in Syrian domestic issues but since this is happening in our neighbourhood, since the Syrian people are our friends and since we wish them well, we cannot be indifferent."
But he said Turkey would have no doubts about defending itself from any attacks on Turkish interests in the country.
Mr Gul said Syria needed broader change than merely removing Mr Assad from office.
"It's not one person - it's the regime, it's the Baath party, it's the structure," he said.
He said that already some 10,000 Syrian refugees had flooded across the border into the country and that Turkey would continue to offer assistance to others.
But he said that, for humanitarian reasons, Turkey had no plans to stop providing electricity to its neighbour.
Meanwhile, in Syria, a bus carrying Turkish pilgrims, came under gun attack in the central Syrian city of Homs, injuring at least two people.
A driver of another bus in the convoy said the bus was carrying 25 butchers back from Saudi Arabia following the festival of Eid al-Adha, or the feast of sacrifice, which starts after the Hajj (pilgrimage).
