Turkey shot down a Syrian government warplane on Tuesday over northwest Syria, where fighting has intensified in recent days, bringing Turkish and Russian forces close to direct conflict in the battle over the last swathe of Syria still held by rebels.

It was the third Syrian warplane Turkey has shot down since Sunday in an escalating campaign against President Bashar al-Assad's forces. NATO-member Turkey supports the rebels, while Assad relies on his superpower ally Russia.

With more than a million refugees amassing since December on the Turkish border, the battle for Syria's Idlib province has brought what the United Nations fears might be the worst humanitarian crisis of the nine-year-old Syrian civil war.

"This relief operation has been overwhelmed. There needs to be more of everything. The first thing is money," UN Under-secretary General for Humanitarian Affairs Mark Lowcock told reporters at a trans-shipment point for supplies in southern Turkey.

Fighting was raging north of the strategic crossroads town of Saraqeb, recaptured on Monday by Syrian troops, one of several times the town, which controls access to Idlib city and Aleppo, has changed hands in recent weeks.

Syrian state media said the army was now combing the town and had dealt heavy blows to fighters still holed up in hideouts on its outskirts. A state television correspondent said Turkey was firing artillery to halt the government advance.

Rebels said the government was aided by thousands of Iranian-backed Lebanese and Iraqi militiamen brought from other areas to help storm the town after two days of failed attempts.

A Syrian general who has defected to the opposition, Ahmad Rahhal, said a Russian announcement on Monday that it had deployed military police in Saraqeb was aimed at blocking Turkey from trying to help rebels reclaim the town.

Putin, Merkel speak; Merkel tells lawmakers she supporters safety zones

German Chancellor Angela Merkel has told fellow conservative lawmakers that she is in favour of setting up safety zones in northern Syria where Turkey is in a deepening standoff with Russia, two participants at the meeting told Reuters on Tuesday.

Merkel also criticized Russian President Vladimir Putin for refusing to take part in a four-way meeting with Turkish President Tayyip Erdogan and French President Emmanuel Macron to de-escalate the Syrian crisis, the parliamentary sources added.

The RIA news agency cited the Kremlin as saying earlier on Tuesday that Putin discussed the situation in Syria's Idlib region with Merkel in a telephone call.

Humanitarian crisis

Turkey has sent thousands of troops and armored vehicles into northern Syria over the past month to fight back against Assad. Last week, a Syrian air strike killed at least 34 Turkish soldiers in the deadliest attack on the Turkish army in decades.

Moscow, which has anti-aircraft missiles in Syria, has since warned Turkey that it cannot guarantee the safety of Turkish planes in Syrian skies.

The Turkish Defence Ministry said on Tuesday that its forces had shot down a Syrian L-39 ground attack jet. Syria's state-run SANA news agency confirmed the plane had been shot down over Idlib province by missiles fired from Turkish warplanes.

Turkish President Tayyip Erdogan and Russia's Vladimir Putin are due to meet on Thursday to seek ways to avert conflict. Asked about the prospect of direct clashes with Turkey, Kremlin spokesman Dmitry Peskov told reporters: "We hope that we're able to absolutely minimise this risk thanks to the close contact between the two countries' militaries."

Turkey, already home to 3.6 million Syrian refugees, says it cannot take any more. It wants to push Assad's forces back to lines agreed in a 2017 deal brokered with Russia and Iran, which left a buffer zone in northern Syria near its border.

Since last week, Turkey has thrown open its frontiers with Greece and Bulgaria to allow migrants to enter the EU, a move apparently aimed at putting pressure on European countries to back it in Syria.

Some 10,000 migrants have tried to cross into Greece by land in recent days and more than 1,000 have arrived by sea at Greek islands, creating fears of a repeat of the 2015-2016 migration crisis, when more than 1 million people crossed into Greece and 4,000 drowned in the Aegean.

The opposition says Syrian government forces are deliberately attacking civilians to provoke them to flee.

A rocket attack believed to have been fired by the Syrian army on a residential quarter of Idlib city left at least nine civilians dead, including five children, according to Osama Idlibi, a rescuer in the opposition-run Syrian Civil Defence.

Overnight Russian and Syrian jets killed at least 10 people in the town of Al Foah in what residents said was a spike in strikes on several towns, including Binish and the outskirts of Tatanaz in Idlib province.

Russia and its Syrian army ally deny indiscriminate bombing of civilian areas and say they target jihadists.

U.S. Ambassador to the United Nations Kelly Craft, in Turkey inspecting the relief efforts, announced $180 million in additional funding for the humanitarian crisis in Idlib.

"Humanitarian aid is only a response but the solution is an immediate ceasefire," Craft told reporters. "This is not something that just happened. This is planned by the Assad regime," Craft said. "It is cruel and brutal."