ISTANBUL—Fierce fighting in Syria poured over two borders on Monday, all but dashing hopes for a United Nations-backed peace plan and spotlighting the conflict´s potential to ensnare neighboring states.

Clashes between Syrian soldiers and rebel fighters along Syria´s border with Turkey wounded at least four people, including two Turkish officials, when Syrian bullets hit a Turkish refugee camp, authorities said. Two Syrians died and 21 were injured in related conflicts on the border, close to the camp.

Hours later, a cameraman for a Lebanese TV station was shot dead by Syrian troops while he worked in northern Lebanon near the border with Syria, said Lebanon´s prime minister, Najib Mikati.

The U.S. said it was "absolutely outraged" by the cross-border attacks. Turkey condemned Monday´s clashes and said Tuesday´s deadline for Syria to withdraw troops from cities and towns was now "void." Mr. Mikati called for the prosecution of those responsible.

The grim news appeared to throw international diplomatic efforts to solve the bloody crisis further into disarray, with analysts predicting Syria´s near certain failure to meet Tuesday´s U.N.- imposed deadline. A full cease-fire was scheduled to begin Thursday.

That action was part of a six-point plan by Kofi Annan, the U.N.-Arab League envoy to Syria, for breaking the deadlock.

The plan had already appeared to stall on Sunday after Syria´s president, Bashar al-Assad, demanded a new condition that wasn´t in the Annan plan: written pledges that "armed groups" first lay down their weapons. They swiftly refused.

The cross-border clashes came amid reports of new attacks on cities across Syria which left more than 100 dead on Monday, including more than 30 in Hama city, said activists.

In the Turkish incident, shots fired from Syrian troops penetrated the perimeter of the camp at the border town of Kilis, in the first such cross-border event.

Scores of Syrians ran out of the camp at Kilis to come to the aid of refugees being fired upon as they attempted to cross the border in an area which had seen clashes that morning, Turkey´s foreign ministry said. In the ensuing melee, shots were fired which went into the camp, wounding five. Other shots killed two and wounded 21 Syrians outside the camp, the ministry said.

Turkish television showed chaotic scenes of men ripping holes in the corrugated iron walls of the camp and running toward the border, while others clambered onto observation towers to try to survey the scene.

Turkey´s foreign ministry summoned Syria´s chargé d´affaires in Ankara to demand troops stop shooting.

State Department spokeswoman Victoria Nuland said the acts were "just another indication that the Assad regime does not seem at all willing to meet the commitments that it made to Kofi Annan."

U.S. officials were mum on what they would do next, other than await the deadline and confer with allies. Ms. Nuland suggested Syria´s actions could prompt Turkey to invoke the mutual defense provisions of the North Atlantic Treaty Organization. Turkey didn´t comment on that.

White House press secretary Jay Carney expressed pessimism that the Annan plan would hold, but said the administration´s opposition to arming the opposition hasn´t changed. "We do not believe it´s at this time the right approach because the further militarization there would potentially have negative consequences," Mr. Carney said.

The Obama administration announced this month that it would share communications equipment with the Syrian opposition. U.S. officials have also said Washington might pass intelligence to Syrian fighters.

U.N. Secretary-General Ban Ki-moon said he "deplores today´s fatal cross-border shootings from Syria into Turkey," and demanded that Damascus "immediately cease all military actions against civilians and fulfill all of its commitments" made to Mr. Annan.

Many Western nations have expressed skepticism about Mr. Assad´s readiness to implement Mr. Annan´s plan. But the Western governments that dominate the Friends of Syria group this month agreed to support the plan given Moscow´s support for Mr. Assad and the apparent lack of appetite for intervention.

Analysts said that a collapse of the Annan plan, sponsored by the Arab League as well as the U.N., would likely see Western and Arab governments return to the U.N. Security Council to broker a tougher Syria resolution.

But there are few signs that Russia or China—which have both vetoed two Western-backed resolutions at the Security Council—would reverse their opposition to censuring or sanctioning the Assad regime.

China on Monday reiterated its call for all sides to implement the U.N. plan, including "troop withdrawal promises." Syrian Foreign Minister Walid Moualem flew to Moscow on Monday, where Russia´s deputy foreign minister said the Kremlin was "working actively with Damascus in order to begin a political settlement process in [Syria]," according to the state news agency.

The potential for a replay of the diplomatic logjam which has frustrated efforts to strengthen measures against Damascus at the U.N. could throw the spotlight onto Turkey, analysts said.

Ankara has long said it is considering, among other options, setting up a "safe" or "buffer" zone along the border with Syria, although ministers have repeatedly sought to stress that the prospect remains some way off.

But in recent days, Ankara has said it might have to act to prevent a humanitarian disaster in the event of a flood of refugees, massacres of civilians by Syrian troops near its border or an incident creating a "risk to national security."

Fleeing Syrians have been mainly housed in Turkey´s southern provinces of Kilis, Gaziantep and Hatay. Ankara has ordered more accommodation constructed in Sanliurfa province.

Monday´s attacks come as Turkey has seen a record influx of refugees and could lead to a hardening of its position.

"What will be important is whether this will remain as an isolated incident or whether this will be part of a recurring security risk," said Sinan Ulgen, a former Turkish diplomat now at the Carnegie Endowment for International Peace. "If Assad does not deliver, then this incident will allow Turkish policy makers to increase their anti- Assad rhetoric and to try to convince the international community to be more assertive toward the Assad regime," he said.

Ms. Nuland said she "would not be surprised" if Turkish officials raised the issue of the cross-border shooting with NATO allies, but added she did not know whether it would fall under the treaty´s collective defense article. The Sept. 11 attacks marked the first and only time NATO ever invoked the mutual defense provision.

Turkish officials say they are increasingly worried. Turkish Foreign Minister Ahmet Davutoglu on Friday called on the international community to take "solid steps" to prevent attacks on civilians. "If the number of immigrants continues to rise, then the U.N. and some other countries should also step in," he told reporters in Ankara.

Lebanon has seen several incursions of the Syrian military since the start of the uprising, though Monday´s death is thought to be the first time a member of the media has been killed there. But Monday´s incidents inside Turkish territory are likely to underline the government´s concern that Syria´s bloody uprising could again spill across their border with Syria.

—Jay Solomon and Carol E. Lee in Washington, Joe Lauria in New York and Nada Raad in Beirut contributed to this article. (Copyright © Dow Jones & Company, Inc.) 04/10/12)