Huge plumes of smoke billowed into the sky as Turkey pressed ahead with its assault against Kurdish forces in northeastern Syria, pounding the region with air raids and artillery fire amid heavy fighting that sent panicked civilians on both sides of the border fleeing.

Tens of thousands of people in Syria scrambled to escape the violence, according to the United Nations, as the Turkish offensive against the Kurdish-led Syrian Democratic Forces (SDF) continued at a fierce pace for a second day on Thursday.

Footage posted online showed residents fleeing with their belongings loaded into vehicles, and some even escaping on foot, while aid agencies warned that an estimated 450,000 people living within five kilometres (three miles) of the border were at risk.

Al Jazeera's Charles Stratford, reporting from the Turkish border town of Akcakale, said much of Turkey's "intense bombardment" was concentrated on Tel Abyad, a town right on the other side the frontier that had been without electricity for the last 24 hours.

In response to the Turkish bombardment, a barrage of shells from the Syrian side rained down on Akcakale, where the streets were practically empty.

"We've had loudspeakers being used by the army to tell civilians to stay in their homes," Stratford said. As evening set in, he added, the situation appeared to be relatively calm after "a day of intense shelling" and reports of families fleeing to seek safety.

Launched on Wednesday, the Turkish ground and air assault began three days after US President Donald Trump announced that his country's troops would be withdrawn from their positions near the border alongside the SDF, the main ally of Washington in the fight against ISIL and a group that has expanded its control in northern and eastern Syria amid the chaos of Syria's eight-year war.

But Ankara sees the presence of the SDF near its border as a major security threat as the group is spearheaded by the Kurdish People's Protection Units (YPG). Turkey considers the YPG a "terrorist organisation" and an extension of the outlawed Kurdistan Workers' Party (PKK), which has waged a decades-long bloody armed campaign for autonomy in Turkey.

In a Twitter post on Thursday, the Turkish defence ministry said that 174 "terrorists" had been killed so far in the offensive, dubbed "Operation Peace Spring". The number, which could not be independently verified, included 19 alleged fighters killed in an air raid on a YPG shelter in Ras al-Ain.

Meanwhile, both Turkish and SDF officials reported civilian casualties. SDF mortar and rocket fire killed at least six civilians in Turkey's southern Mardin and Sanifurla provinces, according to the provincial governors' offices, while Turkish attacks left at least nine civilians in SDF-controlled parts of Syria dead, the group said.

Syria: No talks with SDF

Thursday's developments came as a senior official of the government of Syrian President Bashar al-Assad ruled out Damascus holding any talks with the SDF, accusing the group of pursuing a separatist agenda that gave Turkey a pretext to violate Syria's sovereignty.

Asked about whether the government should resume dialogue with the Kurdish-led forces, Faisal Maqdad, the Syrian deputy foreign minister, said the "armed groups had betrayed their country and committed crimes against it".

"We won't accept any dialogue or talk with those who had become hostages to foreign forces ... There won't be any foothold for the agents of Washington on Syrian territory," Maqdad told reporters in his office in Damascus.

A Syrian Kurdish official said earlier this week that the Kurdish-led authorities in northern Syria may open talks with Damascus and Russia, a military backer of Assad alongside Iran, to fill a security vacuum in the event of a full withdrawal of US forces from the northeastern border region.

UN Security Council divided

The cross-border offensive has opened one of the biggest new fronts in years in Syria's long-running war that has drawn in major international powers, prompting UN chief Antonio Guterres on Thursday to say it was "absolutely essential" for the conflict to de-escalate.

Amid the rising concerns, the United Nations Security Council's five European members - the UK, France, Germany, Belgium and Poland - called on Turkey to "cease" its operation following a closed-door session in New York.

"Renewed armed hostilities in the northeast will further undermine the stability of the whole region, exacerbate civilian suffering and provoke further displacements [of people]," they said in a joint statement, delivered by Juergen Schulz, Germany's deputy ambassador to the UN.

Moments earlier, Kelly Craft, Washington's ambassador to the UN, had said the US did not in "any way" endorse Turkey's operation and warned Ankara it would face repercussions for any over-reach.

"Failure to play by the rules - to protect vulnerable populations and guarantee ISIL cannot exploit these actions to reconstitute - will have consequences," Craft said after the council's meeting.

The 15-member body failed to agree on a joint statement, however, with Russia's UN Ambassador Vassily Nebenzya telling reporters that any such statement on Syria would have to address broader issues related to the country's war, not just the Turkish offensive.

Al Jazeera's James Bays, reporting from the UN, said the comments reflected "clear and deep division" in the body over Syria.

"We did not hear a statement from the president of the Security Council … because if he's going to read out words at the end of one of these closed meetings every Security Council member has to sign on to those words," Bays said.

Erdogan re-issues refugee threat

Turkish President Recep Tayyip Erdogan says the goal of the operation is to create a buffer zone freed of the Kurdish fighters within which some of the 3.6 million refugees currently residing in Turkey can also be resettled.

On Thursday, Erdogan defended the operation and warned the European Union against labelling it an "invasion", renewing his threat to "open the gates" and send the Syrian refugees hosted by Turkey into the bloc if it did so.

Separately on Thursday, Erdogan's Foreign Minister Mevlut Cavusoglu told reporters Ankara would take responsibility for the thousands of suspected ISIL fighters being held by the SDF in makeshift jails across northeastern Syria.

"If Daesh (ISIL) camps or prisons are in the safe zone, we are responsible," Turkish Foreign Minister Mevlut Cavusoglu told reporters on Thursday, adding that Ankara would ask the home countries of foreign ISIL fighters residing in the envisaged zone to take them back.

Several international powers have warned Turkey's operation puts at risk the progress made in battling back the armed group, which at its peak controlled large swathes of Syria and neighbouring Iraq.