The shooting killed two Syrian refugees and wounded at least 23 people, including a Turkish police officer, near the southern Turkish town of Kilis.
Residents of the camp reached by telephone said bullets had ripped through their prefabricated shelters, shattering windows and spreading panic. "Even the camp isn't safe anymore," said Ahmad, a refugee who used only one name out of concern for repercussions in Syria.
A senior Turkish Foreign Ministry official said Syria's ranking diplomat had been summoned and given a harsh message about Turkey's "irritation" with the episode. There was no immediate comment from Syria.
"It is not sufficient reason for military action by itself," said Cengiz Candar, the dean of Turkish analysts of Arab affairs. "But the likelihood of Turkish military action is not as dim as it was a week ago." The shooting will undoubtedly be used to buttress Turkey's call for a buffer zone inside Syria to protect civilians.
Turkey was once one of the closest allies of President Bashar al-Assad's Syrian government. But Prime Minister Recep Tayyip Erdogan became an early and harsh critic of Syria's crackdown on protesters last year, and Turkey has since taken in thousands of fleeing Syrians — as well as rebel leaders and members of ragtag militias that have gathered under the banner of the Free Syrian Army.
On a broader front, diplomatic efforts also faltered. After agreeing to a peace plan negotiated under United Nations auspices, the Syrian government on Sunday announced new conditions for withdrawing its forces from major population centers by Tuesday and carrying out a cease-fire by Thursday. The conditions boiled down to written guarantees from rebel groups and their main backers — Turkey, Qatar and Saudi Arabia — that the opposition would be dismantled first.
Pressure from Moscow, which publicly endorsed the truce and is Syria's crucial international patron, was considered critical to the survival of the peace plan, which was negotiated by Kofi Annan, the special envoy of the United Nations and the Arab League. Walid al-Moallem, the Syrian foreign minister, was scheduled to meet his Russian counterpart in Moscow on Tuesday.
China, which has also vetoed international efforts to isolate Syria, criticized the new conditions. Syria has undermined every truce plan it ostensibly accepted since the uprising started in March 2011.
Mr. Annan considered the plan still alive and was waiting to see what Syria did this week, said Ahmad Fawzi, his spokesman. "To say now that you want written guarantees is a condition that was never in the agreement," Mr. Fawzi said. "The deadline is still valid, and the plan is still on the table."
Mr. Annan was scheduled to head to Turkey on Tuesday to visit some of the nearly 25,000 Syrian refugees there, several thousand of whom arrived in recent days, before moving on to Iran to try to persuade another important Syrian ally to back his plan. Senators John McCain of Arizona and Joseph I. Lieberman of Connecticut were also scheduled to meet with refugees in Turkey on Tuesday.
The Syrian National Council, the main opposition umbrella group, said it was ready to respect a cease-fire.
"The opposition, including the Free Syrian Army, says it will comply if the regime does," said Bassma Kodmani, a member of the Syrian National Council's executive committee, adding that opposition groups within Syria were ready to put a cease-fire into effect on Tuesday and not wait until Thursday. She called the government's actions "clearly not a sign of their intention to cooperate in good faith."
In another ominous sign, deadly violence has intensified in the days leading up to the cease-fire deadline, with opposition groups reporting scores killed on Monday alone in besieged cities and towns across Syria. In Aleppo, which has been less turbulent than much of the country, 10 security officers were shot dead quelling a demonstration and 11 were wounded, along with several civilians, the official Syrian Arab News Agency reported. It said 25 security men were buried nationwide Monday.
Neil MacFarquhar reported from Beirut, and Sebnem Arsu from Ankara, Turkey. Hala Droubi contributed reporting from Beirut.
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