Six human rights defenders have petitioned the Supreme Court to challenge the expulsions, which are being carried out despite warnings that Sri Lanka is in breach of its international obligations, lawyer Lakshan Dias told AFP.
"The case is coming up on September 29," Dias said. "We feel that some of the people who are being sent back face real danger. They have life-threatening conditions if they go back. That is why local rights defenders decided to file this case."
Sri Lanka's Court of Appeal, which is one step lower than the Supreme Court, earlier this month overturned an order suspending the deportations and cleared the way for the authorities to continue expulsions.
The UNHCR said Colombo had initially responded to its appeals and released 71 Pakistanis and two Iranians earlier this month, but 102 people -- 38 Pakistanis and 64 Afghans -- remained in custody and fresh arrests were taking place.
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The refugee agency has maintained that the deportations go against the principle of no forced return, or non-refoulement, enshrined in international law.
The agency has also noted that returning an individual to a country where they face the risk of torture is prohibited under the UN's Convention Against Torture, which Sri Lanka has ratified.
Hundreds of Pakistani Christians and Afghans fleeing persecution in their countries have been arriving in Sri Lanka seeking UNHCR protection in the capital Colombo.
Sri Lanka's foreign ministry also accused the UNHCR of processing asylum claims too slowly, and of not taking responsibility for repatriating those whose refugee claims were rejected.