At least 42 people have been killed in violence across Syria, including 20 rebel fighters battling the Islamic State of Iraq and the Levant group in a suburb of the northern city of Aleppo, a monitoring group said.

The UK-based Syrian Observatory for Human Rights on Sunday said that at least 11 people were killed and 14 others injured in rocket attacks by opposition fighters in a government-controlled neighbourhood in Aleppo.

Also in Aleppo, government air strikes on rebel-held areas left two civilians dead and several others injured, the observatory reported.

At least eight have been killed and several others have been injured after government helicopters dropped a barrel bomb on a residential building in the old city of Aleppo.

Meanwhile, at least 20 rebels were killed in clashes against ISIL in Um al-Qura village located in northern Aleppo suburbs, activists told the observatory.

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The rebel fighters were killed as they attempted to prevent ISIL's advance in the area. Intense fighting broke out between rebels and ISIL on May 31 when ISIL took over the nearby town of Sawran.

Activists also accused the government of dropping 16 barrel bombs in the south of Idlib and six barrel bombs on the central city of Hama

Elsewhere in Syria, a number of Kurdish fighters were killed and others were injured after an ISIL suicide attack, the observatory reported.

The bomber detonated himself in the headquarters of the Kurdish forces behind the Hadaya hotel in central Qamishli.

Meanwhile, in Syria's southwestern Quneitra province, rebel groups have advanced as they captured a checkpoint between the towns of Hader and Beit Jinn.

Rebel groups aim to control Quneitra and advance towards areas in western Damascus suburbs, which have been under government crackdown for the last three years.

Urge for UN intervention

In a letter addressed to the UN Security Council, more than 70 countries have demanded that Syria stop its air strikes and use of barrel bombs that have killed thousands of people.

The letter, which was sent on Thursday to Ban Ki-moon, the UN chief, and the General Assembly, expresses "outrage" at the continuing violence against civilians, especially by the "systematic use of barrel bombs".

"May 2015 was reportedly the deadliest month of the Syrian crisis so far. In recent weeks helicopters of the Syrian air force repeatedly bombarded heavily populated areas in and around Aleppo, leaving hundreds of civilians dead and dozens of others wounded," the letter said.

The countries urged the UN to recall all violations and abuses committed in Syria, adding that they may amount to war crimes and crimes against humanity.

The Syrian conflict has left over 230,000 people killed and millions displaced since its outbreak in 2011.