Double suicide bombings at a coffee shop in the northern Lebanese city of Tripoli have killed nine people and wounded more than 30, officials said.

The army said the attack struck a cafe in the predominantly Alawite Jabal Mohsen neighbourhood at around 7:30pm (17:30 GMT) on Saturday.

"Nine people were killed and 37 others wounded," the Health Ministry said in a statement.

The Nusra Front - Syria's al-Qaeda branch, fighting the regime of President Bashar al-Assad - claimed responsibility for the attack via Twitter.

"A cafe belonging to the [Alawite Arab] Democratic Party in Jabal Mohsen was targeted with a double martyrdom attack, to avenge the Sunnis in Syria and Lebanon," read the tweet.

The Arab Democratic Party is the main group representing the Alawite minority in Lebanon.

The last few months have been relatively quiet in Tripoli, where Alawites of Jabal Mohsen have in the past clashed with Sunnis from the Bab al-Tabbaneh neighbourhood as the two communities support rival sides in Syria's war.

Gunfights between the two districts in recent years have killed scores of people. Saturday's bombing, however, was the first of its kind.

'Terrorist act'

Lebanese politicians and movements condemned the attack, branding it a "terrorist" act and calling for unity.

"This crime will not terrorise the Lebanese or the residents of Tripoli, and it will not weaken the government's resolve to confront terrorism and terrorists," Prime Minister Tammam Salam said in a statement.

The powerful pro-Syrian Shia Hezbollah movement blamed "takfiri [extremist] terrorists" for carrying out the attack, in a reference to radical Sunnis.

Security sources said on Sunday that the army had detained three suspects from the Sunni Mankoubeen neighbourhood, adjacent to Jabal Mohsen, over links to the suicide bombers.

The National News Agency named the attackers as residents of Mankoubeen.

Lebanon has seen a series of attacks and suicide bombings since the conflict in Syria began nearly four years ago. Saturday's attack was among the deadliest to hit the country in the past year.

Lebanon's al-Qaeda-linked group, the Abdullah Azzam Brigades, has claimed responsibility for some of the attacks. It has warned that attacks will continue as long as Hezbollah participates in Syria's civil war alongside Assad's military.

The last suicide attack struck in Lebanon on September 20, when a suicide car bomber attacked a Hezbollah checkpoint near the eastern town of Khreibeh.

Since October, the army has deployed heavily in Tripoli, detaining hundreds of people in an attempt to stem the violence.

Lebanon's Alawites have generally sided with Assad, who belongs to their sect.

People in Bab al-Tabbaneh support Syrian rebels, who like the Syrian population, are mostly Sunni.