A year of sanctions, diplomacy and harsh rhetoric failed to stop Syria's bloody crackdown and oust President Bashar Assad. With frustration running high, Turkey and other countries that have staked moral credibility on ending the violence are increasingly looking at intervention on Syrian soil, a strategy they have so far avoided for lack of international consensus and fears it could widen the conflict.

Diplomacy has not yet run its course, but more treacherous options, including aid to Syrian rebels, are likely to come up at a meeting of dozens of countries that oppose Assad, including the United States and its European and Arab partners, in Istanbul on April 1.

One prominent option floated by Turkey is a "buffer zone" on the Turkish-Syrian border, which could amount to a foreign military occupation, intent on regime change even if the aim is humanitarian in name. The risks of such an endeavor in a combustible region are evident in the U.S.-led invasion of Iraq, Israel's occupation of southern Lebanon decades ago and Syria's own military presence in Lebanon until 2005.

Yet, comparisons with international hesitation over the Balkans bloodshed in the 1990s make it ever harder to engage in seemingly endless, and fruitless, diplomacy.

Turkish Prime Minister Recep Tayyip Erdogan discussed Syria with U.S. President Barack Obama on Sunday at a nuclear security conference in South Korea, and said it was not possible to tolerate events there. Earlier, Erdogan was asked by reporters on his plane whether a safe zone inside Syria was on the agenda.

"Studies are under way," Erdogan said. "It would depend on developments. The 'right to protection' may be put into use, according to international rules. We are trying to find a solution by engaging Russia, China and Iran."

Erdogan predicted that "everything could change" if those countries withdraw their support for Syria, and he accused Assad of reviving ties with and "protecting" rebels of the PKK, a Turkish Kurd group at war with the Turkish state. Turkey already hosts some 17,000 Syrian refugees, and casting the Syrian crisis in terms of Turkey's national security strengthens the case for intervention.

U.N. and Arab League envoy Kofi Annan was discussing Syria on Sunday in Russia, which vetoed a U.N. Security Council resolution aimed at pressuring Assad but has shown increasing impatience with him. His next stop is Beijing, which also blocked U.N. action.

Annan's plan, endorsed by the U.N. Security Council, includes a cease-fire by Syrian forces, a daily two-hour halt to fighting to evacuate the injured and provide aid, and inclusive talks about a political solution.

But, there are still questions about how such an agreement would be overseen and enforced. An Arab League monitoring effort in Syria failed, labeled a farce by some who participated. The likelihood that a Syrian regime that has shelled cities would talk in good faith to the people it targeted is remote, and outgunned Syrian rebels say the time is long past for any negotiation.

The United Nations says more than 8,000 people have died. Many were civilian protesters.

Assad bucked the trend of relatively quick transitions to new governments in regional uprisings. Tunisia, Egypt and Libya, where a NATO bombing campaign helped oust Moammar Gadhafi, did not bear the same geopolitical tensions as the Syrian case. The conflict there comes as Israel considers a plan to bomb the nuclear facilities of Iran, a regional power and close ally of Assad, and further destabilization in Syria could set off lasting unrest.

Turkey and the United States, in an election year, "are reluctant to make more forceful moves because of the long-term costs of policing the sectarian violence that will surely happen following the collapse of the Assad regime," said Arda Batu, professor of international relations at Yeditepe University in Istanbul and editor-in-chief of the Kalem Journal, a website about regional affairs

The countries meeting in Istanbul hope to help the Syrian opposition coalesce into a more coherent movement that can show all Syrians, not only the majority Sunni Muslims, that they would have a place in a post-Assad future.

The "Friends of Syria" group of more than 60 countries made little headway at its maiden meeting in Tunisia in February, and countries are already talking about creating a subgroup to discuss military options more urgently. Saudi Arabia and Qatar are some of the strongest advocates of this approach.

One idea sees Arab countries and Turkey _ with the U.S., ideally, but possibly without _ establishing a buffer zone along the Syrian-Turkish border that would serve as a humanitarian corridor and staging ground for the rebel Free Syrian Army. On the Syrian side of the border, it would entail army defectors and other guerrillas wresting control of land and holding it, which they have been unable to do.

Earlier this month, CIA chief David Petraeus met Erdogan in Ankara. Turkish media said the prime minister warned that deepening instability in Syria would provide a "living space" for militant organizations active in the region, including the PKK.

On Saturday, Turkey's Yeni Safak newspaper, which is considered close to the government, said 500 military personnel have inspected areas close to the border for a safe zone that could stretch 20 kilometers (12.5 miles) inside Syria, and would end their "studies" before the meeting in Istanbul.

The newspaper did not provide sources, but the report contributed to a sense that the safe zone idea is slowly gaining traction despite the pitfalls.

"If the U.S. is not involved, there is no way Turkey would get involved in it," said Osman Bahadir Dincer, a Syria expert at the International Strategic Research Organisation, a center in Ankara, the Turkish capital. However, he predicted "some kind of an intervention in the form of a buffer zone or a safe zone" within one or two months.

Dincer said a decision to arm the Free Syrian Army was unlikely at the Istanbul meeting amid questions over the composition of the ragtag militias, and divisions between fighters in Syria and the Syrian National Council, the opposition group based outside the country.

"The opposition is too fragmented, there is confusion as to which group represents who, or what they represent," he said.

The U.S. and other key allies, however, are considering providing Syrian rebels with communications help, medical aid and other "non-lethal" assistance. Ben Rhodes, the White House deputy national security adviser for strategic communication, said in South Korea on Sunday that communications assistance could be critical to the opposition's efforts.

If any military intervention is to gain the international legitimacy that was accorded the Libya mission, it will need the U.N.'s stamp of approval. That requires the acquiescence of veto-wielding Security Council members Russia and China, an unlikely possibility that could only occur if they are included in the process and feel similarly betrayed by the Assad regime.

Without the U.N., the U.S. would be stretched to justify military involvement. It could help NATO ally Turkey in the event of a Syrian attack across the border, or make a U-turn on a doctrine of caution about intervention that Obama has insisted on since he was a presidential candidate.

"Of course, it is not possible to remain a spectator, to wait and not to intervene," Erdogan said in South Korea, with Obama at his side. "It is our humanitarian and conscientious responsibility. We are engaged in efforts toward doing whatever is necessary within the framework of international law. We are happy to see that our views on this overlap."

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Associated Press writers Suzan Fraser in Ankara and Bradley Klapper in Washington contributed.