Tension escalated on Monday between Turkey and Syria after shots fired from across the border injured six people in a Turkish refugee camp, angering Ankara a day before a visit by international envoy Kofi Annan.
Four Syrians and two Turks were wounded amid clashes between rebels and Syrian army forces on the other side of the border, the governor of Kilis province where the camp is located told Anatolia news agency.
"Ricocheting bullets from the Syrian side hurt four Syrians and two Turks -- a police officer and a translator -- on duty at the camp," governor Yusuf Odabas said, adding that the injuries triggered protests among the Syrians in the camp.
It was the first time Syrian fire from across the border had hurt people on Turkish soil, prompting Turkey's foreign ministry to urgently contact the Syrian mission in Ankara to "immediately halt the shootings," a source said.
Washington said it was "absolutely outraged" by the attack, saying "we strongly condemn any attack by the Syrian regime on refugees in bordering countries."
Turkish officials briefing Washington believe "the regime knew that it was firing across the border, that it was pursuing activists and that these were intentional acts," said State Department spokeswoman Victoria Nuland.
Monday's attack came after 21 injured Syrians were brought across the border into Turkey, 17 of them wounded in an overnight clash in their village of Sucu in Aleppo province, close to the border with Turkey.
Two of the wounded died shortly after they arrived, while the rest were under treatment in hospitals in Kilis, the governor told Anatolia news agency.
The incidents caused panic among refugees at the Kilis camp, who often go out to meet fellow countrymen trying to flee but become targets of snipers on the other side of the border.
Turkish security officials blocked exits from the Turkish side into Syria to prevent further injuries, Odabas added.
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It was not immediately clear who fired the shots into Turkey, or whether the group helping the wounded out of Syria was targeted.
Turkey's Foreign Minister Ahmet Davutoglu meanwhile briefed UN chief Ban Ki-moon and Annan on the escalating number of Syrians arriving at the border, some of them with wounds.
Turkish diplomatic sources say Annan will visit refugees at the border on Tuesday, while prominent US senators John McCain and Joe Lieberman are also expected to pay a visit to the camps.
Syrians accompanying the wounded into Turkey said that clashes in Syrian towns on Sunday night left many wounded despite the imminence of a ceasefire supposedly agreed by Damascus with Annan for Tuesday, but they were only able to carry some of them across the border.
Turkish Deputy Foreign Minister Naci Koru was pessimistic about the ceasefire coming into force, saying, "April 10 is invalid, a new phase starts tomorrow."
"It is now quite clear that the Annan Plan will not be delivered, after the assaults that have been raging since yesterday," he added, quoted by Anatolia news agency.
Koru's words backed up a warning from Turkey's prime minister of unspecified measures if Damascus fails to abide by the April 10 deadline to cease its yearlong crackdown on dissent that has left nearly 10,000 people dead.
"We will patiently follow the process until April 10," Prime Minister Recep Tayyip Erdogan said on Saturday.
But "we will implement steps" if violence does not stop after that, he added.
Several scenarios are being floated by the press, including the setting up of a buffer zone along the border to protect the large numbers of refugees.
Turkey has taken in some 4,000 Syrians since last Thursday, pushing up the number of refugees on its soil to more than 25,000.