The family of a Libyan member of parliament and campaigner abducted by armed men two weeks ago fear she may have been subjected to torture and sexual violence.

Seham Sergiwa, who is also a prominent women’s rights activist, disappeared from her home in the eastern city of Benghazi on 17 July and has not been heard from since.

Relatives believe Sergiwa, 56, may have been silenced by forces loyal to Khalifa Haftar, the 75-year-old military strongman who controls much of the east of the country.

Hours before the abduction, Sergiwa, who has has a doctorate in clinical psychology, told an interviewer on a pro-Haftar TV channel that Haftar’s supporters included “extremists”, and called for the Muslim Brotherhood, the major Islamist organisation seen as a principal foe by Haftar, to be included in a unity government.

Sergiwa has repeatedly criticised the continuing offensive launched by Haftar’s self-styled Libyan National Army against Tripoli, the capital and seat of the fragile UN-recognised Government of National Accord (GNA), in April.

Her husband was shot in the legs during the raid and was badly beaten. Her 14-year-old son was also assaulted and injured. Both were in hospital for several days.

“We have had no news since the abduction,” said Sergiwa’s niece Nicole, speaking from Indiana in the US. “We thought it might just be an arrest, even though her family was beaten and shot up. But we are worrying that she is still alive and being hurt in ways we cannot imagine … torture, rape.”

Sergiwa came to prominence as a campaigner against sexual violence in Libya after the fall of Muammar Gaddafi in 2011, and was elected to the country’s new legislative assembly in 2014.

“She is truly an amazing person who speaks her mind. She would never compromise her principles. She does what she thinks is right. She has been speaking out forcefully against Haftar,” her niece said.

The family say masked members of the 106th Brigade, a force commanded by one of Haftar’s sons, were seen at Sergiwa’s home when she was taken away.

Witnesses also described fighters from other armed groups present during the raid, including at least one militia faction with a reputation for brutal treatment of civilians.

The words “the army is a red line” were sprayed on the outside of the house, an apparent warning to other critics of Haftar’s forces.

Fighters loyal to the UN-recognised Government of National Accord load a launcher 25 miles south of Tripoli. Photograph: Mahmud Turkia/AFP/Getty Images

Libya has been plunged into a new crisis by Haftar’s offensive which has so far led to more than a thousand civilian deaths and derailed diplomatic efforts to reconcile two rival armed political factions.

After rapid early gains, Haftar’s forces have stalled as resistance by a coalition of militias fighting for the GNA has hardened.

Jalel Harchaoui, of the Clingendael Institute, a thinktank based in The Hague, said the military setbacks had undermined the authority of Haftar and his forces. One consequence was that the slightest criticism could provoke a very strong reaction.

“Sergiwa is a highly educated woman, clearly someone who has shown a commitment to liberal values and human rights. She is not a crusader against Haftar but wanted to show the world there was a way to be independent … But it is a very, very polarised atmosphere,” Harchaoui said.

A spokesman for the Benghazi-based government’s interior ministry, Tarek El-Kharraz, said an investigation was under way to determine Sergiwa’s whereabouts, but denied any involvement by forces loyal to Haftar in her disappearance.

The UN support mission in Libya has expressed its concern over Sergiwa’s disappearance and called on Libyan authorities to investigate the matter, warning against “unlawful arrest and abduction based on political views”.

Tim Eaton, an analyst at London’s Chatham House, said the abduction was a clear indication that Haftar or his associates would not tolerate dissent about the Tripoli offensive.

Khalifa Haftar, left, with Russia’s foreign minister, Sergei Lavrov, in Moscow in 2017. Photograph: Sergei Savostyanov/Getty Images

“It is a snapshot of an army … that is increasingly intolerant of dissent and unwilling to subject itself to civilian authority,” Eaton said.

“The broader context is that the April offensive was launched when the United Nations secretary general was in Tripoli advocating a political settlement, which shows that Haftar’s forces don’t attach much credibility to the UN. The muted international response to the offensive proved them correct.”

As in Syria and Yemen, the conflict in Libya has been aggravated by regional rivalries. Turkey and Qatar have broadly supported western forces, while the United Arab Emirates, Saudi Arabia and Egypt have backed the east, with growing tacit support from the French. Italy, France and the UN have run often competing peace processes, sometimes reflecting commercial interests.

The French government, the European power closest to Haftar, insists it had no prior warning of his assault.

Analysts say the outcome could shape not just the politics of Libya, but also the security of the Mediterranean, and the relevance of democracy across the Middle East and north Africa.

A statement from the family described Sergiwa as “a moderate and progressive woman” who had returned from the UK to help her country.

“The silencing of women and democracy in favour of dictatorship should not be tolerated. Above all, Seham is not only a politician, but a mother, daughter, sister and wife. We desperately want to welcome her home alive,” the statement said.