​​​​​​Concern over a weeks-long hunger strike by hundreds of undocumented migrants in Belgium's capital has mounted this week after four men stitched their lips shut to stress their demands for legal recognition and access to work and social services.

Aid workers say that more than 400 migrants, holed up at two Brussels universities and a baroque church in the heart of the city, stopped eating on May 23 and many are now very weak.

Many of the migrants, who are mostly from South Asia and North Africa, have been in Belgium for years, some for more than a decade, but say their livelihoods have been put at risk by COVID-19 shutdowns that led to the loss of jobs.

"We sleep like rats," said Kiran Adhikeri, a migrant from Nepal who worked as a chef until restaurants closed because of the pandemic. "I feel headaches, stomach pain, the whole body is full of pain."

"I am begging them (the Belgian authorities), please give us access to work, like others. I want to pay taxes, I want to raise my kid here, in this modern city," he told Reuters, gesturing from his makeshift bed to where fellow hunger strikers lie listlessly on mattresses in the crowded room.

Many looked emaciated as health workers cared for them, using saline drips to keep them hydrated and tending to the lips of those who sewed their mouths shut in a bid to show they have no say over their plight.

The Belgian government said it will not negotiate with the hunger strikers over their plea to be granted formal residency.

Junior minister for asylum and migration Sammy Mahdi told Reuters on Tuesday the government would not agree to regularise the status of the 150,000 undocumented migrants in Belgium, but is willing to hold talks with the strikers on their plight.

"Life is never a price worth paying and people have already gone to the hospital. That's why I really want to try to convince all persons and all organisations behind it to make sure they don't give a false hope," Mahdi said, when asked about the hunger strikers.

"There are rules and regulations ... whether it is around education, whether it is around jobs, whether it is around migration, politics needs to have rules."

Europe was caught off guard in 2015 when more than a million migrants made it to the bloc's shores, overwhelming security and welfare networks, and fomenting far-right sentiment.

The European Union has proposed an overhaul of the bloc's migration and asylum rules to ease the burden on Mediterranean-shore countries, but many governments would rather tighten borders and asylum laws than accommodate new arrivals.