As Russian and Syrian forces prepare an offensive to take the last remaining rebel stronghold, some refugees from Idlib province say Syrian rebels are stopping their families from fleeing the fighting.

Key points: Syrian refugees in Lebanon fear for their families in Idlib, as final offensive looms

Syrian refugees in Lebanon fear for their families in Idlib, as final offensive looms Syrian Arab Army has allowed corridors for refugees to leave, but the rebels won't let them

Syrian Arab Army has allowed corridors for refugees to leave, but the rebels won't let them Russia is threatening to "eliminate" the rebels, but observers expect them to make their final stand

"We were forced to exile from our homes and our country because of the terrorists of Jabhat Al Nusra and Daesh [Islamic State]," one 53-year-old refugee, Ahmad, said.

He said he had been in Lebanon's Bar Elias camp in the Bekaa Valley for five years. All the refugees in this part of the camp are from Idlib province, where Russian and Syrian forces are preparing an offensive against the last stronghold held by rebels.

Ahmad said he was frightened for his family remaining in Idlib.

"Right now there are no heavy bombardments as they are negotiating, they are meeting," he said.

"But of course if the terrorists refuse to surrender, the Syrian Arab Army will advance and conquer Idlib. There is no other option and we back the Syrian Arab Army on that."

Aid agencies say a new offensive could displace around 800,000 people. ( ABC News: Adam Harvey )

The United Nations and the United States, among other nations, are urging the Syrian Government and its allies Russia and Iran to negotiate a peaceful solution in Idlib … but these refugees say the onus is on the rebels.

"They won't let people leave to safer zones, to safer regions," Ahmad said.

"The Syrian Arab Army has opened more than one corridor but the terrorists are not allowing anyone to leave because they are using them as human shields."

International NGOs are far more critical of the regime and its allies.

Lama Fakih from Human Rights Watch in Lebanon said an offensive to take Idlib will inevitably lead to civilian casualties.

Sorry, this video has expired White Helmets pull survivors from buildings destroyed by airstrikes in Idlib (Photo: Supplied/Hobait Media Office)

"We've seen that the [Syrian] Tiger Forces, which have been responsible for previous offensives like Eastern Ghouta, has deployed to Idlib so we are anticipating the same kinds of violations that have characterised Government offensives in other parts of the country," she said.

"These are things like unlawful restrictions on humanitarian aid deliveries, indiscriminate bombings that have targeted hospitals and schools, as well as the use of prohibited weapons like cluster bombs and even chemical attacks."

In the Bekaa Valley camp, another Idlib refugee, a 28-year-old who is also called Ahmad, said he was terrified for his sister and her children.

"My sister couldn't escape as the terrorists are preventing civilians from leaving as they use them. If they'd allow them to leave they would leave," he said.

"They could have come here, somewhere else, gone to Damascus, but they wouldn't let them leave."

Even if they were able to leave Idlib, refugees from the fighting can no longer come to Lebanon or Turkey.

Both nations have closed their borders after taking an estimated 4.5 million Syrian refugees.

Aid agencies said a new offensive could displace around 800,000 people.

Opposition forces in Idlib say they will fight to the end for control of the province. ( AP: Ugur Can/DHA )

They will likely head towards the closed Turkish border. Turkey does not want that and is urging a negotiated ceasefire. Russia's Syria envoy Alexander Lavrentiev says his nation will "eliminate the rebels".

"Our position is very strict and very tough. There should be no negotiations with them," he said.

Asked what he meant by "eliminate", Mr Lavrentiev replied: "If they surrender, that would be the best decision."

A surrender seems unlikely.

Moderate groups may consider it, but the trapped rebels include thousands of fighters linked to jihadist groups like the renamed Al Nusra Front.

Observers say those fighters are bound to make Idlib their last stand.