A jailed outspoken American-Cambodian activist has begun a one-week hunger strike to protest conditions in prison ahead of US President Joe Biden's visit to the kingdom.

Lawyer and campaigner Theary Seng, a long-time critic of Cambodian Prime Minister Hun Sen, was imprisoned for six years for treason in June in a mass trial that also saw around 60 opposition figures convicted.

Her strike comes ahead of Biden's visit to capital Phnom Penh later this week to attend the Association of Southeast Asian Nations (ASEAN) summit.

The US leader is also expected to meet with Hun Sen.

In a statement on Monday, the 51-year-old activist's team "calls upon the President to press Hun Sen for Theary's freedom".

Jared Genser, a US-based lawyer representing her, said she had also been denied rights permitted to other prisoners "including having weekly access to Church services and the ability to regularly make phone calls".

Currently in a prison in western Preah Vihear province, Theary Seng began her hunger strike Monday in a bid to compel authorities to transfer her to Prey Sar Prison in Phnom Penh.

Meanwhile, in a further push to raise awareness, six Cambodian youth activists launched a week-long hunger strike Monday, roughly 30 kilometres away from Theary Seng's prison.

"We hope government leaders and especially US President to discuss this matter with Hun Sen," group representative Hun Vannak told AFP.

The 70-year-old prime minister is Asia's longest-serving leader, with critics alleging he has wound back democratic freedoms, used the courts to stifle opponents, and jailed scores of opposition activists and human rights defenders.

Hun Sen has vowed to run for office again next year and has supported his eldest son Hun Manet to succeed him in the future.

AFP