A Greek MP has joined Syrian refugees in a hunger strike outside the Athens parliament to protest against the government's refusal to help them move to other European countries to reunite with relatives.

Yiannis Michelogiannakis, a deputy from the leftist opposition Syriza party, said he decided to strike after an activist group said two protesters had died, one of a heart attack and one while trying to cross the border into Albania.

A lawmaker's duty is to be here on the streets, on the side of the weak, not inside the luxurious parliament. Yiannis Michelogiannakis

Their deaths could not be independently verified. The government said on Tuesday it could not help the refugees because they had not applied for asylum or shelter.

About 300 Syrian refugees have camped outside parliament since November 19 demanding the right of passage to other European countries and some have started a hunger strike.

The migrants refuse to register because that, according to European Union law, would mean they would have to stay in Greece as the first EU country they entered as refugees.

"A lawmaker's duty is to be here on the streets, on the side of the weak, not inside the luxurious parliament," the deputy, a 51-year-old doctor, told Reuters.

Michelogiannakis added that he was also striking over the 2015 budget due to be voted on Sunday and new EU/IMF austerity measures to ensure an early bailout exit for Greece.

He was expelled from the Socialist PASOK party after voting against the bailout last year and joined Syriza.

"I'm determined to stay here opposite parliament with the Syrians. I will pop in tomorrow to vote against the bailout budget and I'll come back," he said. "I'm determined to stay until the ordeal of the Syrians ends."

About 46,500 Syrians have arrived in Greece since 2011, more than half of them this year, and most want to move on to other EU countries where they have relatives, according to the interior ministry. It is not known how many have slipped across Greek borders to make their way further north illegally.