TRIPOLI, Lebanon — A bomb ripped through a bus carrying civilians and members of the Lebanese military during this morning’s rush hour in the northern city of Tripoli, killing 18 people and wounding 46, security officials said.

The officials said the dead included 10 off-duty soldiers, from a bomb planted on the side of a main street that detonated as the bus passed. The streets were filled with people heading to work, which contributed to the many casualties, the officials said.

The blast was the first significant bombing in Lebanon in months and comes as the country is making moved to put three years of back-to-back crises behind it.

President Suleiman of Lebanon was heading to Syria later today in the first visit there by a Lebanese president since 2005, aimed at patching up troubled ties between the neighboring countries. A day earlier, parliament approved a long-awaited national unity government between the Western-backed factions of Prime Minister Fuad Saniora and Iranian- and Syrian-backed Hezbollah.

Some officials said the Tripoli bombing may have been the work of Al Qaeda-inspired militants seeking revenge for the military’s assault last year on their bastion in a nearby Palestinian Arab refugee camp, Nahr el-Bared. Hundreds were killed in the monthslong battle that ended with the militant Fatah Islam group fleeing the camp.

The army described the blast as a “terrorist attack targeting the army directly.” A senior military officer told The Associated Press that at least 13 people were killed, including 11 soldiers. He spoke on condition of anonymity for the same reason. The discrepancy with the higher police figures could not immediately be explained.

He and the security officials spoke on condition of anonymity because they were not authorized to speak to the media.

Mr. Saniora vowed the attack “will not affect the launching of our government.”

“Lebanon and the Lebanese will not kneel … or submit to the criminals and the terrorists,” Mr. Saniora said. He refrained from linking the blast to Syria, as he had done in previous bombings, many of which targeted anti-Syrian figures in Lebanon.

Syria’s Foreign Ministry denounced the explosion and expressed support for Lebanon “against the hands that try to disrupt its security and stability.”

Shattered glass littered central Tripoli’s Banks Street in Tripoli’s center. The small public bus, which had been bringing passengers from the remote Akkar region, home to many military members, was riddled with shrapnel. Soldiers used sniffer dogs to search nearby parked cars, as forensic experts sifted through the bus wreckage.

The bomb was locally made, packed in a bag with nuts and bolts to maximize casualties, and was detonated by remote control, security officials said.

An electrician, Hatem Hussein, 24, said he ran to the scene after hearing the loud explosion. “The wounded were lying on the ground, men in military uniforms,” he said.

Another witness, Khaled Bizri, 38, said he didn’t have the “courage to look at the dead,” who included a popular bread vendor well known among residents, named Abu Ayman. “Everybody knew him. This was his place for 30 years.”

Tripoli, about 50 miles north of Beirut on the Mediterranean coast, is Lebanon’s second-largest city and has a mostly Sunni Muslim population dominated by groups loyal to the Western-backed parliamentary majority.

Despite a relative calm elsewhere, it has in the past weeks witnessed sectarian clashes between Sunni fighters and followers of the Alawite sect, an offshoot Shiite sect, that killed and wounded dozens of people.

A former Prime Minister, Omar Karami — a prominent politician from Tripoli — said it is too early to know the motive, but said the attack could be linked to the 2007 Nahr el-Bared violence, given the high casualties among soldiers. Fatah Islam claimed responsibility for a May 31 bomb blast that killed a soldier in Abdeh, near Tripoli.

Lebanon has seen a series of explosions in the last 3 1/2 years, including a 2005 truck bombing that killed a former Prime Minister, Rafik Hariri, in Beirut, an explosion that sparked the political and security upheaval in the country.

But there have been no serious attacks against politicians or public places since February.