20 Syrian soldiers killed in Aleppo explosion

At least 20 Syrian soldiers and militia were killed Saturday in a fresh attack by rebels, who planted explosives in a tunnel under an army position in Aleppo, a monitor said.

The old city area of Aleppo has seen horrific violence ever since a major rebel offensive in July 2012.

It is a flashpoint area that sees daily fighting, and the army has set up multiple positions there.

"Islamist rebels detonated a tunnel near the Zahrawi market in the Old City of Aleppo, killing at least 20 army soldiers and pro-regime militiamen," said the Syrian Observatory for Human Rights.

Fighting broke out after the explosion, and at least one rebel was killed, said the Britain-based Observatory.

The attack was claimed by the Islamic Front, Syria's largest rebel alliance, which groups thousands of fighters across the strife-torn country.

The Front posted a link to a video on its Twitter account, showing a huge blast throwing a massive cloud of debris up into the air.

In recent weeks, the Islamic Front has frequently used tunnels to plant massive amounts of explosives beneath army positions.

The tactic has been used mainly in Aleppo and neighbouring Idlib provinces.

The latest blast came a day after the Observatory said some 2,000 people have been killed since January in regime bombing of rebel-held areas of Aleppo city and nearby towns and villages.

Syria's war has killed more than 162,000 people, and forced nearly half the population to flee their homes.

Six journalists killed in Syria in May

Meanwhile, the London-based watchdog Syrian Network for Human Rights announced on Saturday that six jounalists were killed and 13 others were wounded across Syria in May. 

One of the journalists was killed after being tortured by the Syrian army, while another was hung by militants of the Islamic State of Iraq and Levant (ISIL), the organization said. 

As the Syrian conflict entered its fourth year, the country has become a deadly place for journalists who are regularly kidnapped and murdered, human rights groups say.

According to the watchdog, a staggering 328 journalists, including 12 foreign journalists, were killed by the Syrian regime since the beginning of the civil war in 2011. 

In a statement released on May 3, the network added that Assad's forces has also arrested 848 journalists, 19 of which died after being tortured.