The United Nations Secretary General has strongly condemned car bombings outside two mosques in the northern Lebanese city of Tripoli, which killed 42 people and wounded more than 500.

A statement released shortly after the explosions hit worshippers following Friday noon prayers, said Ban Ki-moon "strongly condemns the explosion of two car bombs ... leaving tens killed and hundreds injured".

He also "calls on all Lebanese to exercise restraint, to remain united, and to support their state institutions, particularly the security forces, in maintaining calm and order in Tripoli and throughout the country," it said.

The United States also condemned the apparently coordinated bombings, the biggest and deadliest in Tripoli since the end of Lebanon's civil war.

US National Security Adviser Susan Rice wrote on Twitter that Washington "strongly condemns" the attacks.

Rice also extended condolences for "the loss of innocent life".

Meanwhile, EU foreign affairs chief Catherine Ashton said she was "appalled" by the attacks and called for a swift investigation.

Ashton "condemns this terrorist attack in the strongest terms and reaffirms that terrorism and any use of violence against civilians are completely unacceptable," a statement from her office said.

"She calls for a swift investigation into the events and to bring the perpetrators to justice."

French President Francois Hollande strongly condemned the "odious, cowardly attacks".

Hollande pledged France's continued support for Lebanon "in this tragic context", and backed the efforts of President Michel Sleiman and the Lebanese army to "safeguard Lebanon from the consequences of the Syrian crisis."

Britain's foreign office also condemned the "abhorrent attacks", with the foriegn office minister, Alistair Burt, saying "it is important that the Lebanese authorities are given the support they need to investigate this attack fully and return calm to the streets of Tripoli."

Source: Agencies