Two female migrants found dead in a mountainous rural region of Bulgaria died due to freezing conditions, authorities said Sunday, as asylum seekers continue to try to reach the EU despite harsh winter weather.

The women were part of a group of 19 migrants, including 11 children, found Saturday by border police near the southeastern town of Malko Tarnovo. Their nationality was still unclear.

Migrants passing through Bulgaria on their way to western Europe are subject to violence, threats and even police dog attacks, according to a damning new report released by anti-poverty agency Oxfam Friday.

The report, which was based on interviews with over 100 migrants arriving in Serbia from Bulgaria, said they suffered "extortion, robbing, physical violence, use of weapons, threats of deportation, and police dog attacks."

Bulgaria's ruling conservative party of Prime Minister Boyko Borisov scored a resounding victory in local elections this weekend, winning mayoral seats in 22 out of the 27 major cities, official results showed Monday.

Borisov's Citizens for European Development of Bulgaria (GERB) party won the mayor's seat in the second largest city of Plovdiv in Sunday's second-round ballot, a week after securing the capital, Sofia.

Europe's migrant crisis took a dark turn late Thursday when an Afghan man seeking to cross from Turkey was shot dead by Bulgarian border guards, shortly before the EU and Turkey struck a deal to stem the refugee influx.

The killing is thought to be the first of its kind in Europe's worst refugee crisis since World War II.

The truck with 71 dead migrants abandoned in August on an Austrian motorway had Hungarian plates and a Slovakian poultry firm's livery. But five of the six suspects since arrested are Bulgarians.

To detectives probing the dark business of human trafficking, this was no surprise. 

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.

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.

Bulgaria began on Thursday deploying up to 1,000 troops to buttress its southeastern border with Turkey as hundreds of migrants remained stranded for a third day on the Turkish side, a senior official said.

"We have triggered a plan for the start of the step-by-step deployment of up to 1,000 troops along the whole Bulgarian-Turkish border (within the next week)," interior ministry chief of staff Georgy Kostov said.

Austrian authorities said Tuesday police have made two more arrests over the gruesome discovery last week of 71 dead migrants in an abandoned truck on an Austrian motorway.

One man was arrested in Bulgaria and a second in Hungary, said Verena Strnad, spokeswoman for prosecutors in the area where the truck was found last Thursday.

A plane carrying Bulgarian Prime Minister Boyko Borisov made an emergency landing on Sunday because of a problem with the landing gear, the second such incident involving the leader in six months, the transport ministry said.

The undercarriage failed to retract when the plane left the airport at the eastern town of Varna, the ministry said in a statement.