Twin car bombs claimed by jihadists on a government-held district of Syria's third city Homs killed at least 100 people, mostly civilians, a monitoring group said Wednesday, sharply raising an earlier toll.

Tuesday's attack was the deadliest of its kind in Homs since the Syrian conflict erupted three years ago, and came as government forces make a new attempt to overrun the handful of remaining rebel enclaves in the city centre.

It also came a day after President Bashar al-Assad registered to stand in a controversial June 3 election which is expected to return him to office despite the raging violence.

Al-Qaida's Syria affiliate, Al-Nusra Front, said it carried out the twin bombings against the city's Abbasiyeh neighborhood, which is mainly inhabited by members of Assad's Alawite minority community.

"God allowed the Al-Nusra Front's fighters to achieve a feat despite draconian security measures," the hardline Sunni group said.

"It is so that they (the residents of pro-government areas) taste the hell that our brothers have tasted," it added, referring to the repeated bombardment by the army of rebel-held districts of Homs.

The Syrian Observatory for Human Rights had initially reported a toll of at least 51 dead and scores wounded from the bombings but raised that Wednesday to at least 100 dead.

Observatory chief Rami Abdel Rahman said the bombers had not come from inside the city's rebel enclaves but from elsewhere.

"It's a message from the rebels to Assad that there won't be any safe zones to hold the election in," he said.

Also on Tuesday, mortar fire killed 14 people in Damascus, the state SANA news agency reported, blaming rebels in the outskirts of the capital.