Al-Qaida-affiliate al-Nusra Front reiterated threats to target Hizbullah's stronghold, accusing it of supporting the regime of Syrian President Bashar Assad and committing atrocities against Sunnis in Syria.

“The assaults that Hizbullah is committing against Sunnis in Syria are not a secret anymore... Our target at this stage is to attack the Iranian party's strongholds,” al-Nusra Front said via Twitter.

The dialogue between Hizbullah and al-Mustaqbal was in limbo after the two parties' officials exchanged accusations, a few days before a new round of talks between them.

Over the weekend, Hizbullah officials Hashem Safieddine and MP Mohammed Raad lashed out at the March 14 alliance's formation of the National Council.

Three British teenagers suspected of trying to travel to Syria were released on bail late Sunday after being questioned for 24 hours on suspicion of preparing acts of terrorism, police said.

Two 17-year-old boys from northwest London and a man aged 19 were detained by authorities in Turkey on Friday and were deported to Britain on Saturday, where they were promptly arrested by counter-terrorism officers.

Three British male teenagers have been detained in Turkey and prevented from travelling to Syria, police said Sunday.

Two are aged 17 and from northwest London while the third is 19 years old, and all three are being held by Turkish authorities.

The United States will have to negotiate with Syrian President Bashar Assad to end the civil war now entering its fifth year, Secretary of State John Kerry conceded in an interview that aired Sunday.

"Well, we have to negotiate in the end. We've always been willing to negotiate in the context of the Geneva I process," Kerry said in an interview carried out Saturday.

At least 18 people were killed and more than 100 injured in Syrian government air strikes near the capital Damascus on Sunday, a monitor said.

The raids on the town of Douma, northeast of the capital, came as the conflict entered its fifth year and the death toll soared over 215,000.

More than 215,000 people have been killed in Syria in four years of conflict, a monitoring group said on Sunday as the brutal civil war entered its fifth year.

The Britain-based Syrian Observatory for Human Rights said it had documented the deaths of 215,518 people in Syria since March 2011, among them more than 66,000 civilians.

The Vatican's ambassador to the United Nations has endorsed military action against the Islamic State group in Iraq and Syria -- an unusual move because the Vatican traditionally has opposed force in the region.

In an interview with the U.S. Catholic website Crux, Archbishop Silvano Tomasi said IS fighters were committing atrocities on a huge scale and the world needed to intervene.

An agent who helped three British schoolgirls cross into Syria to join the Islamic State group was also working as a courier to transfer money to jihadists, a Turkish newspaper reported on Sunday.

Turkey announced Thursday that it had arrested an intelligence agent working as a spy for an unidentified country in the U.S.-led coalition and said he was a Syrian national.

Syria's conflict enters its fifth year on Sunday with the regime emboldened by shifting international attention and a growing humanitarian crisis exacerbated by the rise of the Islamic State group.

More than 210,000 people have been killed and half of the country's population displaced, prompting rights groups to accuse the international community of "failing Syria".