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.

British Foreign Secretary William Hague on Wednesday pressed the world's chemical weapons watchdog to finish its probe into the alleged use of chlorine gas in Syria as urgently as possible.

The Organisation for the Prohibition of Chemical Weapons announced Tuesday it is sending a fact-finding mission to Syria after claims that President Bashar Assad's forces may have unleashed industrial chemicals on civilians.

Rebels in southern Syria say they've united tens of thousands of fighters and rejected the extremism and infighting that have plagued the uprising elsewhere, but still want for external support.

The so-called Southern Front was created around two months ago and includes some 30,000 fighters from more than 55 mainstream rebel groups operating from the Jordanian border to the outskirts of Damascus and the Golan Heights, the rebels say.

Iran on Tuesday defended Syria's June presidential election, saying the vote could end the three-year-old civil war ravaging its close ally.

The response came as President Bashar Assad registered to stand in the June 3 vote, which is widely expected to return him to office despite the grinding conflict.

The world's chemical watchdog announced Tuesday that it is sending a fact-finding mission to probe the recent alleged use of chlorine gas in the Syrian conflict.

The head of the Organization for the Prohibition of Chemical Weapons, Ahmet Uzumcu, announced "the creation of an OPCW mission to establish facts surrounding allegations of use of chlorine in Syria," a statement said.

Four new candidates have submitted applications to run for Syria's presidency in elections that are expected to return President Bashar Assad to power next month, the parliamentary speaker said Tuesday.

Mohamed al-Lahham announced the names of the four new candidates, among them a woman and a Christian, in a session of parliament in the capital Damascus.

Mortar shells slammed into central Damascus on Tuesday, killing at least 14 people and wounding 86, as a blast and a rocket attack left at least 45 people dead and 85 hurt in Homs.

"Fourteen citizens were killed and 86 others wounded by terrorists who targeted the Shaghur neighborhood in Damascus with four mortar shells," the SANA news agency said, adding that two shells hit the Badr al-Din al-Hussein institute.

An international rights group called Tuesday for the U.N. Security Council to block the flow of weapons to Syria's regime, which it said was launching indiscriminate attacks with "barrel bombs".

The New York-based Human Rights Watch said it had documented 85 aerial attacks by the regime against opposition areas in northern Aleppo province since February 22 alone.

Since soldiers arrested and beat Abu Noor, his son and nephew at their modest house in Baghdad's Adhamiyah neighborhood, he and his wife have been too scared to leave home.

"I feel sick when I talk about this... I only go to work and I come back," said the 54-year-old, who was too scared to give his real name, remembering the night six months ago when the soldiers arrived.

The United Nations said on Monday that Jordan's third refugee camp, Azraq, had received its first group of Syrians ahead of its official opening this week.

"A few hundred Syrian refugees started to arrive in Azraq camp this morning," Ali Bibi, spokesman for the U.N. refugee agency UNHCR in Jordan, told AFP.