Chinese police on Monday released at least three of five feminist activists detained for over a month in a case which prompted an international diplomatic outcry, one of their lawyers said.

Wei Tingting, Wang Man and Zheng Churan were "released on bail", lawyer Liang Xiaojun cited family members as saying. The release indicates that the trio are unlikely to face criminal prosecution.

He added that there was "no clear news," about fellow activists Wu Rongrong and Li Tingting, even as a deadline for prosecutors to formally charge them was due on Monday.

Police detained the activists -- all aged 32 or under -- shortly before International Women's Day last month as they were preparing to hand out leaflets about sexual harassment on public transport.

The five women had been linked to several stunts in different Chinese cities aimed at highlighting issues such as domestic violence and the poor provision of women's toilets.

The three released activists "will still have restrictions on their freedom," effectively preventing them from engaging in further activism, said Liang, who represents Wu.

"In the eyes of the police, they are still suspects... they will need to regularly update authorities on their whereabouts," Liang said.

Police originally told lawyers the activists were suspected of "picking quarrels and provoking trouble", a vague charge increasingly used by authorities under President Xi Jinping to detain and jail protesters for holding small-scale demonstrations.

They later changed the charge to "illegal assembly," which carries the same maximum punishment of five years imprisonment.

China's ruling Communist Party does not tolerate organised opposition, and often clamps down on small activist groups.

But the detentions were seen by rights groups as unusually harsh given the small scale of the women's stunts and the fact that they had been praised in China's state-run media.

Police interrogations of the women -- several of whom suffer from chronic health problems such as asthma and an unspecified heart condition -- have focused on a 2012 stunt named "Occupy Men's Rooms", their lawyers said.

Chinese criminal lawyers said that given the date of their detention, prosecutors had to formally charge them by the end of Monday, or police would be obliged to release them.

- 'Right to speak out' -

Their detention prompted condemnation from rights groups as well as the United States and the European Union, which called for their release.

Britain-based group Amnesty International said that China must "immediately and unconditionally release the five women if they are being detained solely for peacefully exercising their rights to freedom of expression and association."

US Secretary of State John Kerry urged Beijing on Friday immediately to free the women, with similar comments made by Vice President Joe Biden.

"Each and every one of us has the right to speak out against sexual harassment and the many other injustices that millions of women and girls suffer around the world each and every day," Kerry said in a statement.

Prospective presidential candidate Hillary Clinton tweeted last week that the detentions were "inexcusable" and "must end".

China rejected the US calls, and said on Monday that it had made "representations" to Washington on the issue.

"We urge the United States to respect China's judicial sovereignty and stop interfering in China's domestic affairs," foreign ministry spokesman Hong Lei told a regular news briefing.

Ten of the activists' parents and spouses at the weekend issued an eleventh-hour plea to authorities for their release.

The activists were "young, kind-hearted, and full of a sense of responsibility to society", the group wrote in a letter to Beijing prosecutors that was posted online Saturday.

They added: "Supporting gender equality and the interests of women is no crime!"