This week's bloodshed on the Israeli-Lebanese border may be a harbinger of what lies ahead if Syria decides to retaliate for Israeli airstrikes.

Syria has so far responded to Israel in the U.N. Security Council. But if it is attacked again, Syria could play its trump card — south Lebanon, a zone bursting with Islamic guerrillas supported by the Syrians and willing to fight Israel. That could invite Israeli bombings that have devastated the south in the past.

Tensions along the frontier have heightened since Israeli warplanes struck a base for Palestinian militants near Damascus on Sunday, the first such foray deep inside Syria in three decades.

The next day, an Israeli soldier on patrol at the border with Lebanon was killed by gunfire. Hours later, a Katyusha rocket (search) apparently intended for Israel landed just short of the border, killing a 5-year-old boy in this southern Lebanese village. Another rocket landed in a field.

On Wednesday, Israeli warplanes flew over Lebanon breaking the sound barrier, but did not attack anywhere.

The Israel-Lebanon front has been largely dormant since Israeli troops pulled out of an occupied zone in 2000. But concern over escalation prompted U.N. envoy Terje Roed-Larsen (search) to urge all parties Wednesday to exercise restraint and avoid the "steep precarious path toward more violence."

Israeli Prime Minister Ariel Sharon has threatened to take the war against Israel's enemies anywhere, and cross-border attacks from Lebanon likely would bring retaliation.

Syria is the main power broker in Lebanon, stationing 20,000 soldiers there and wielding enormous influence with the government. It also backs the Hezbollah (search) guerrillas who control southern Lebanon and are known to have hundreds, perhaps thousands, of Katyusha rockets.

Hezbollah, which has threatened to retaliate against attacks on Lebanon by firing rockets into northern Israel, urged Lebanese to be prepared for all possibilities. It pledged in a statement "unqualified commitment to one battle and fate with steadfast Syria."

For now, Syria has limited its reaction to Sunday's attack to taking a complaint to the United Nations, basking in a swell of support from the Arab world and international criticism of Israel - except from the United States.

Syria's military is seen by experts as no match for Israel's modern, well-trained army, but the government in Damascus has suggested it has other means to respond.

"We are not a superpower, but we are not a weak state either. We're not a country without cards," President Bashar Assad told Al Hayat newspaper on Tuesday.

His foreign minister, Farouk al-Sharaa, added a warning in a letter to the Security Council: "Syria is not incapable of creating a resisting and deterring balance."

A diplomat in Damascus said that might mean heating up the Lebanese-Israeli border if Syria is attacked again.

"If there's another occurrence, they'll have to do something, depending on the attack," said the diplomat, who spoke on condition of anonymity.

Patrick Seale, an expert on Syrian affairs, expects Assad to continue focusing on diplomatic action, but added that another attack might force Syria to do more.

While it's in no one's interest to escalate on the Lebanese-Israeli border "there must be a good deal of anger over what happened" Sunday, Seale said by telephone from Paris.

In Houla, the village several miles from where the Israeli soldier was killed Monday, mourning and not politics dominated at the Yassin residence Wednesday.

Five-year-old Ali Yassin was killed in his sleep when a rocket hit the first-floor bedroom of the family's house on the last Lebanese hill before the border with Israel. His 4-year-old brother, Ahmed, suffered serious injuries.

"The shell came from the west, meaning from Lebanon," their grandfather, Kamel Yassin, said as he received condolences in the house's garden. Upstairs, shrapnel holes and blood splatters dotted the walls of the boys' bedroom.

Maj. Gen. Lalit Tewari, the Indian commander of U.N. peacekeepers in southern Lebanon, said he had urged all parties to respect international rules on avoiding hitting civilians.

But, he told Yassin: "What I tell the parties cannot bring the boy back."