The first signs of the predicted mass movement of Syrian civilians seeking to escape an imminent joint Russian-Syrian air assault emerged on Monday when the UN said 30,000 of the 3.5 million people in the opposition-run enclave of Idlib had already been internally displaced.

The UN has warned that Idlib could turn into the worst humanitarian disaster of the 21st century as hundreds of thousands of civilians surge to the currently closed Turkish border to evade an assault led by the regime of President Bashar al-Assad.

Russia and the US also accused each other’s allies of preparing to mount chemical attacks. Donald Trump’s national security adviser, John Bolton, stated on Monday that the United States, Britain and France had agreed that another use of chemical weapons by the Syrian government would result in a “much stronger response” compared with previous air strikes.

“We’ve tried to convey the message in recent days that if there’s a third use of chemical weapons, the response will be much stronger,” Bolton said when fielding questions after a policy speech.

“I can say we’ve been in consultation with the British and the French, who joined us in the second strike, and they also agree that another use of chemical weapons will result in a much stronger response,” he said.

British sources say there is a credible threat that the Assad government could launch a new chemical weapons attack and that the US and its allies believe that the explicit threat of a serious counter-attack may be the only way to prevent it.

After a chemical weapons attack in the Damascus suburb of Douma in April, the US, UK and France launched more than 100 cruise missiles, but no attempt was made to hit Syrian or Russian air bases.

Any new strike on Syria would be of “a different order to anything that has happened before,” said one UK official.

A three-way summit in Tehran on Friday, including Russia, Syria and Turkey, failed to agree on a ceasefire. The Turkish president, Recep Tayyip Erdoğan, is now putting maximum diplomatic pressure on Russia – with US backing – not to mount an all-out military assault that would create chaos and reduce the chances of a political settlement.

But in a sign that the slowly building military pressure has already triggered a mass movement, the UN office for the coordination of humanitarian affairs spokesman David Swanson reported 30,542 people had already been displaced from the north-west of the country by Sunday. The majority have headed close to the border with Turkey.

A displaced Syrian girl stands outside her tent in a camp in Kafr Lusin. Photograph: Aaref Watad/AFP/Getty Images

Turkish media reported a further 17 civilians had been wounded in bombing raids in the Idlib region on Monday. The Turkish defence minister, Hulusi Akar, demanded an end to the attacks and again called for a ceasefire.

In Geneva, the UN special envoy Staffan de Mistura was negotiating privately with Turkish, Iranian and Russian officials as part of a two-day effort to get the countries to seek political and not military solutions.

The Russian president, Vladimir Putin, has to decide if he is willing to ignore Erdoğan and allow the Assad regime to attack as many as 60,000 Syrian opposition fighters, of which around 10,000 are not allied to Turkey, but instead follow the UN proscribed jihadist group, Hayat Tahrir al-Sham (HTS), an offshoot of al-Qaida’s former branch in Syria.

Erdoğan is trying to persuade Putin to look at other options. Turkey says it is still possible HTS can be persuaded to dissolve, but this is unlikely.

The shifting alliances and rivalries among the opposition fighters makes it hard for Turkey to impose a solution. Mark Lowcock, OCHA director, acknowledged that there were many rebels and fighters from “terrorist” groups in the province, but stressed that “there are 100 civilians, most of them women and children, for every fighter in Idlib”.

The opposition accuses Russia and its allies of already striking at hospitals and civil defence centres to force rebels to surrender in a repeat of earlier, large-scale military offensives.

In the House of Commons, the UK foreign office minister Alistair Burt refused to give an undertaking that MPs would be given a vote if the UK saw a need to join the US in mounting reprisal raids for what it adjudges to have been a chemical attack by the Syrian government.

The shadow foreign secretary, Emily Thornberry, urged the UK government not to take any action in response to HTS-controlled areas “until the chemical weapons inspectors, the OPCW, have visited those sites under the protection of the Turkish government, independently verified those reports and attributed responsibility for any chemical weapons used. Relying on so-called open source intelligence provided by proscribed terrorist groups is not an acceptable alternative”.

In practice, the OPCW would take many months to reach its conclusions, assuming the Syrian government cooperated.