Immigration authorities in Malaysia have detained Mu Sochua, vice president of the banned opposition Cambodia National Rescue Party (CNRP), four sources with direct knowledge of the matter told Reuters news agency on Thursday.

The detention came before a planned return of exiled CNRP leaders, including Mu Sochua and Sam Rainsy, the party's founder, on Saturday, the independence day of Cambodia.

More:

The sources, one of whom is with the Malaysian government, declined to be named.

Malaysia's immigration department did not immediately respond to Reuters new agency's questions.

Al Jazeera also reached out to the Home Affairs Ministry, but has not received any response.

In late October, Mu Sochua was also denied entry to Thailand and was sent back to Malaysia.

“Malaysia’s immigration’s action to detain Mu Sochua at #KLIA is absolutely ludicrous & unacceptable. She’s done nothing wrong and should be immediately released, and allowed to undertake the consultations she planned with the #Malaysia government and civil society groups" @hrw pic.twitter.com/VDlQ8PY23g

— Phil Robertson (@Reaproy) November 7, 2019

Before boarding her flight from Jakarta to Kuala Lumpur on Wednesday night, Al Jazeera asked Mu Sochua if she was concerned about her safety, and she replied: "If we were to be concerned we wouldn't get to the goal."

There was no response to a follow-up message Al Jazeera sent to Mu Sochua on Thursday morning.

Jerald Joseph, an official of the Malaysian Human Rights Commission, said he had spoken with Mu Sochua in detention, who said she had been told she would not be deported to Cambodia.

It was not immediately clear where authorities would send her next.

Immediate and uncontional release

In a statement issued on Thursday, the ASEAN Parliamentarians for Human Rights (APHR) called for the "immediate and unconditional release" of Mu Sochua.

"The decision to detain her is a complete disregard for ASEAN’s stated commitment to democracy and human rights. We all know that any attempts to block Cambodian opposition members from returning to their countries is solely based on political grounds and is a blatant attempt to silence their voices," said Kasit Piromya, former Thailand MP and Board Member of APHR.

Cambodia's exiled former opposition leader calls for uprising

The news of Mu Sochua's detention comes just a day after Malaysian authorities acknowledged on Wednesday that they have detained two Cambodian opposition activists late on Tuesday.

Cambodia has arrested at least 48 opposition activists this year for allegedly plotting to overthrow the government before the planned return from self-exile of Sam Rainsy, founder of the dissolved CNRP, on Saturday.

Hun Sen's government deployed troops along its borders in response to Rainsy's announcement of his planned return.

Rainsy fled to France four years ago following a conviction for criminal defamation in which he was ordered to pay $1m in compensation. He also faces a five-year prison sentence in a separate case.

He has previously said it was legitimate to seek to topple Hun Sen because the prime minister has created a one-party state and was not prepared to hold free and fair elections.

On Wednesday, Thai Prime Minister Prayuth Chan-ocha said that Rainsy would not be allowed to enter Thailand on his way to Cambodia.

With additional reporting by Leonie Kijewski in Phnom Penh