Sofia (AFP) - Bulgaria began deploying up to 1,000 troops Thursday to buttress its southeastern border with Turkey with hundreds of migrants 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.

The move followed an overnight attempt by 660 migrants to cross into Bulgaria illegally, Kostov said, adding Bulgarian patrols spotted the trespassers and alerted Turkish authorities, who turned them back at the border.

EU member Bulgaria has already sent more than 1,000 extra police to its porous 260-kilometre (160-mile) Turkish border and sealed part of it with a 30-kilometre razor-wire fence that is being extended.

Prime Minister Boyko Borisov approved a plan involving joint army-police border patrols which took effect Thursday morning, Kostov added.

Defence Minister Nikolay Nenchev told public BNR radio the first 50 soldiers were already on their way with another 160 to follow later Thursday.

"We have the capability to dispatch up to 1,000 people at any moment," the minister said.

"You can never predict which direction this refugee wave will take. We do not know in advance where these masses of people will pile up. If one country decides to seal its border, they go to another border. So we are prepared," Nenchev said.

Hundreds of migrants remained stranded for a third day in the northwestern Turkish city of Edirne after police prevented them from reaching nearby border checkpoints with Greece and Bulgaria.

Bulgaria's border police chief Antonio Angelov said Wednesday that the Turkish authorities had committed to turn migrants back to Istanbul but added his service was prepared to intercept them and transfer them over to the migration services for registration.

Bulgaria, which is not a member of the passport-free Schengen zone and borders Romania, Serbia, Macedonia and Greece, has intercepted a total of 7,400 migrants so far this year, mainly Syrians entering illegally from Turkey.

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But migrants have generally sought to bypass the country to avoid its dilapidated refugee camps while awaiting registration and status procedures.

Interior Minister Rumyana Bachvarova told private Nova television Thursday the country would keep registering asylum-seekers rather than letting them continue their journey unhindered.

Bulgarian parliament meanwhile amended the country's penal code, stiffening punishment for people smugglers who now risk up to six years in jail and a fine of up to 20,000 leva (10,220 euros, $11,570) if they help trespassers over the border or ten years jail for transporting migrants plus larger fines.

Law enforcement officials participating in smuggling risk 12 years in jail, fines and a partial or full confiscation of property, parliament ruled.