A new march by migrants trying to reach Europe overland from Turkey was blocked by police outside Istanbul Monday.

Around 700 mostly Syrian men, women and children from a group that had been blocked for the past week at Istanbul's main bus station set out overnight on foot for the northwestern city of Edirne, 250 kilometers (150 miles) away.

More than 100,000 people -- many waving Turkish flags -- attended an "anti-terrorism" rally in Istanbul Sunday, AFP reporters estimated, as President Recep Tayyip Erdogan presses a major offensive against Kurdish rebels. 

The huge crowd thronged Yenikapi Square on the shores of the Marmara Sea for the demonstration, which was to culminate with an address by Erdogan.

Berlin police have warned that there are risks of attacks by Kurdish militants against Turkish targets in Germany, according to an internal document seen by Welt am Sonntag newspaper.

The document classified "for use only by the service" said militants from the Kurdistan Workers' Party (PKK) could carry out "offenses against state institutions including Turkey's consulate and embassy as well as cultural and commercial facilities in the form of media-attention attracting occupations, vandalism and arson."

At least 13 migrants died off the coast of Turkey Sunday after the inflatable dinghy carrying them to Greece collided with a ferry, Turkish media reported.

Four children were among the victims of the accident involving a boat carrying 46 migrants from the northwestern Turkish port of Canakkale to the Greek island of Lesbos, Dogan news agency reported.

Hundreds of mostly Syrian migrants who had been blocked for days by police in the northwestern Turkish city of Edirne while trying to reach the nearby Greek border have agreed to abandon their roadside protest.

A group of around 500 people, who had been engaged in a standoff with riot police on the road leading into the city since Tuesday, lifted their makeshift camp Saturday night and left the area in buses provided by local authorities, an Agence France Presse photographer witnessed.

Nearly 30 migrants were feared missing off the Greek island of Lesbos, the coastguard said Sunday, in the latest boat sinking in an ongoing Aegean Sea tragedy that has cost hundreds of lives.

The coastguard said it had rescued 20 people spotted in the water by a helicopter from EU border agency Frontex, but the survivors said another 26 people had been in the boat.

Thousands are expected to take part in an anti-terror demonstration in Istanbul Sunday, in a rally that will gauge the level of popular support for President Recep Tayyip Erdogan's two-month-old offensive against Kurdish militants.

The "one voice against terrorism" demonstration in Yenikapi Square, which will be addressed by Erdogan, is expected to dwarf a similar rally held in the capital Ankara Thursday in which some protesters railed against the outlawed Kurdistan Workers' Party (PKK), which the president has likened to the extremist Islamic State Group.

Thousands of migrants sought their way through a chaotic maze of rumor and proliferating border controls in the western Balkans on Saturday.

In the latest chapter in the EU's escalating refugee crisis, Croatia, Hungary and Slovenia tussled over how to cope with a wave of refugees desperate to reach northern Europe.

Turkish fighter jets carried out a new barrage of cross-border airstrikes this week against Kurdish militants in northern Iraq, killing at least 55 rebels, state-run Anatolia news agency reported Saturday.

The strikes by F-16 and F-4 jets targeted caves, houses and camps used by the militant Kurdistan Workers Party (PKK), Anatolia said, citing unnamed security sources.

A five-year-old Syrian girl was found dead on Saturday and several other refugees were believed to be missing when their boat sank in an attempted crossing from Turkey to Greece, the state ANA agency reported.

The Greek coastguard rescued another 13 people and was looking for other survivors, the agency said.