Lebanon and Jordan Act on Syria-Related Violence Wounded Syrians being helped onto a truck after an attack by regime forces in Aleppo on Sunday. BEIRUT, Lebanon - Lebanon and Jordan moved aggressively on Monday to squelch the spread of violence from Syria's deadlocked civil war, the most significant register yet of alarm over the strife spilling over Syrian borders. Hussein Malla/Associated Press Sunni gunmen patrolled a street leading to a Shiite neighborhood in Beirut, Lebanon, after a weekend of clashes left at least 4 people dead, at least 20 injured and nearly 50 under arrest. Lebanese Army tanks and armored personnel carriers rolled into the streets of Beirut and Tripoli to stop a night of gunfights as the Lebanese military issued an extraordinary statement urging sectarian and political leaders to refrain from incitement to pull the country back from the brink. "The country's fate is at risk," the statement said. Tension in some areas is increasing to unprecedented levels. In Jordan, the authorities seized a ring of Jordanian extremists suspected of plotting mayhem with munitions from Syria, while Jordanian military skirmishes with suspected Islamic militants traversing the Syria-Jordan border left a Jordanian colonel dead - the first military casualty suffered by Jordan in connection with the Syria conflict since it began 19 months ago. Fears that the Syrian uprising against President Bashar al-Assad could destabilize the Middle East have been growing for months as the conflict has aggravated sectarian tensions that cut across national boundaries and has sent more 300,000 refugees spilling into Turkey, Iraq, Jordan and Lebanon. But those fears escalated sharply on Friday when a large bomb obliterated a Beirut block, killing eight people including Brig. Gen. Wissam al-Hassan, the top Lebanese security official. The blast was followed by a weekend of roadblocks, sporadic street protests and isolated clashes in Lebanon. General Hassan, the head of internal security, was a hero to many Sunni Muslims, Christians and others here for his efforts to expose assassinations and other political meddling by the Syrian government, which is a close ally of the politically dominant Lebanese Shiite militant group, Hezbollah. Mourners at the general's funeral chanted against Mr. Assad and some waved the flags of the Free Syrian Army battling to oust him. Mr. Assad and much of the ruling elite belong to a sect considered an offshoot of Shiism, while the Free Syrian Army is dominated by Sunnis. The proximity of that conflict and the car bombing that killed the general brought back smoldering grudges but also a feeling of dread left from Lebanon's own bloody, 15-year civil war. By Monday night, Lebanese state news media said that at least 4 people were dead, at least 20 injured and nearly 50 under arrest in connection with clashes set off by the Beirut bombing. Three of those killed and most of the injuries were in the northern city of Tripoli near the Syrian border, a flash point with armed groups on both sides of Syria's sectarian divide. In Washington on Monday, a State Department spokesman said that the United States would send an F.B.I. team to help Lebanon investigate the bombing. Although the Hezbollah-dominated government is normally hostile to Washington, Lebanon's chief of Internal Security Forces, General Ashraf Rifi, confirmed in a telephone interview that judicial authorities had accepted the F.B.I."s offer of assistance. He said that "technical experts" from the bureau would arrive within 48 hours to survey the crime scene. In Amman, Jordanian authorities said Monday that soldiers had fought two gun battles overnight against small groups of what the government called Islamist extremists at the Syria-Jordan border, arresting 13 of them. The Associated Press reported that the assailants were trying to cross into Syria. The gun battles came hours after the government said it had arrested a ring of 11 Jordanians accused of planning to use explosives and weapons obtained in Syria for terrorist attacks against the American Embassy, shopping malls and other targets. Samih Maayta, Jordan's minister of media and communication, said the government had evidence that they had traveled to Syria and planned to go back for munitions to use in the attacks. Reporting was contributed by Hwaida Saad and Hania Mourtada in Beirut, Ranya Kadri in Amman, Jordan, and Jodi Rudoren in Jerusalem.