Turkey on Monday slammed U.S. Secretary of State John Kerry for suggesting negotiations would have to be opened with President Bashar Assad to end the conflict in Syria.

Foreign Minister Mevlut Cavusoglu told the state-run Anatolia news agency that all Syria's current problems, on the fourth anniversary of the start of the conflict in March 2011, were caused by the Assad regime.

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.

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.

Turkish President Recep Tayyip Erdogan on Saturday dismissed claims that a new constitution expanding his powers would install a "dictatorship."

"They say the presidential system would bring dictatorship. Is there dictatorship in the United States, Mexico or in Brazil?" Erdogan said in the western province of Canakkale.

Turkish police on Friday detained three people accused of insulting President Recep Tayyip Erdogan and other top officials on Twitter, after raiding their homes.

The raids were the latest in a string of actions against critics of Erdogan on Twitter, as activists express growing alarm over the limits on freedom of expression in Turkey.

Turkey's former president Abdullah Gul said Friday he had no plans to return to politics, ending speculation of a possible comeback in June legislative elections.

Gul -- a co-founder of the ruling Justice and Development Party (AKP) -- has been a key figure in Turkish politics for the last decade, serving as prime minister, foreign minister and president from 2007-2014.

The Turkish government took an unexpected step back on its controversial homeland security bill, taking the legislation out of parliament and sending it back to a committee for further revision.

The move appears to be a concession to the pro-Kurdish opposition in parliament to avoid damaging a fragile peace process with Kurdish rebels.

Three British girls who crossed into Syria to join the Islamic State group were helped by a Syrian working as a spy for a country in the U.S.-led coalition, Turkey's foreign minister said Friday.

Meanwhile, video footage emerged purportedly showing the agent helping the girls into a car in southeastern Turkey on their way to Syria.

The Turkish coastguard opened fire to stop a cargo vessel carrying 337 mainly Syrian migrants heading towards European Union waters and arrested the suspected traffickers, a top local official said Friday.

The coastguard had on Thursday evening launched an operation to chase down the 59 metre (194 foot) Dogan Kartal as it headed through the Dardanelles Straits in northwest Turkey.