Russia has sent in aircraft to attack the Syrian rebel-held province of Idlib in its first major assault for months, as foreign ministers gathered in Brussels to pledge as much $5bn (£3.8bn) to support countries like Jordan, Lebanon and Turkey, where many Syrians have sought refuge.

The bombardment of the country’s last rebel-held enclaveby Russian and Syrian planes was the most extensive yet. A full-scale assault could lead to hundreds of thousands of refugees joining the four million who have already fled the country.

As many as three million people live in Idlib province, including many jihadist fighters who have been transferred to the region after the fall of other rebel enclaves, such as Homs and Aleppo.

In September Turkey persuaded Russia to hold off from a full-scale attack on Idlib, but it appears Moscow is losing patience with Turkey’s efforts to disentangle the main rebel fighting group from the rest of the population. Turkey had promised to remove the militants and their heavy arms from the areas bordering regime-held areas, creating a deconfliction zone.

Russia also has a history of mounting assaults at times when western leaders gather to discuss Syria.

The three-day Brussels conference, culminating on Thursday, is expected to reiterate both that no refugees would be forced to return to Syria against their will and no cash for reconstruction, as opposed to humanitarian aid, would be released unless the president, Bashar al-Assad, agreed to a UN-sanctioned political settlement.

The west concedes Assad will remain in power, but is using the economic leverage of possible reconstruction money to try to persuade Assad to come to terms with the opposition. A new UN special envoy, Geir Pedersen, has relaunched the year-long search for a politically balanced committee to rewrite the Syrian constitution.

Idlib residents said Russian aircraft conducted at least 12 aerial strikes on residential areas, including a civilian prison on its outskirts, where they said dozens of prisoners escaped. At least 10 civilians were killed and 45 injured. More than 100 civilians have been killed since the beginning of the year.

Russia’s defence ministry confirmed it had hit Idlib in coordination with Turkey, targeting drones and weapons stores of the Hayat Tahrir al-Sham (HTS) jihadists it said were intended for use in an attack on a major Russian airbase near the Mediterranean coast.

HTS said in a statement: “There is no solution against the Russian occupier, Assad’s forces and the Iranian militias except confrontation and confrontation only.”

A Russian army base, south of government-controlled Halfaya town, also targeted Kafr Zita in the northern Hama countryside while cluster bombs hit several rebel-held towns in southern Idlib, rebels said.

Meanwhile, in eastern Syria, US-backed fighters besieging the final area of territory occupied by Islamic State said they had pushed further into the enclave following clashes and airstrikes.

The Syrian Democratic Forces said: “Our fighters advanced deep into areas controlled by the terrorist organisation and established a number of new points, following clashes in which terrorists sustained a number of deaths and injuries.”

In Brussels Syrian civil society groups used the first two days of the conference to set out how the Assad regime was wreaking vengeance on those refugees that chose to return. They also warned the hard pressed Lebanese and Jordanian governments, intent on preventing the refugees settling permanently, were trying to force the refugees home before their safety was guaranteed. Jordan houses more than 600,000 refugees, Lebanon 1 million – a sixth of the population – and Turkey has as many as 3 million.

The Lebanese prime minister, Saad Hariri, is leading a divided coalition in which some members are close to Damascus, including the refugee minister, Saleh Gharib. who complained he has been excluded from the government delegation.

Lebanon is divided over the refugees with some complaining they are contributing to power cuts and joblessness.

The UN has launched an appeal for $3.3bn for inside Syria, and $5.5bn is for refugees and host communities in neighbouring countries.

Johannes Hahn, the EU commissioner for neighbourhood policy and enlargement, insisted “refugees need to return under conditions of safety and dignity, when, and only when, it is of their own will.”

Those conditions do not currently exist. Idlib is controlled by a terrorist organisation. Reprisals have taken place in areas retaken by Damascus. Property rights are being actively undermined. There is continuing uncertainty over issues of civil documentation; and no one knows if education certificates will be recognised.

Hahn said: “Damascus appears intent on screening returnees for ‘loyalty’, slowing and manipulating the issue of return. There are no ‘winners’ in this conflict. But we must seek to minimise the number of ‘losers’ in the interest of creating lasting stability in future”. He urged Lebanon to think again about refugees being able to work.

The EU foreign affairs chief, Federica Mogherini, is determined to maintain a political edge to the conference by insisting that refugee return would be impossible without a political settlement, including accountability for war crimes. She said: “The EU is determined to avoid that this becomes a forgotten war and a forgotten conflict. We are entering a different phase of the conflict. We risk entering a zone of no war, no peace; that is not a sustainable basis of the country to go forward.”

Reuters contributed to this report