Syrian government tanks crossed the frontline in the battleground city of Aleppo for the first time in four years, as a Russian-backed offensive to retake the rebel-held east escalated on the ground.

Pro-government forces were “gradually advancing” after street battles on Tuesday in the divided city’s rebel-controlled neighbourhoods, according to the UK-based Syrian Observatory for Human Rights.

Tuesday’s offensive came as Russia said it has sent an advance missile system to the Syrian port city of Tartus on the Mediterranean coast. The S-300 was a defensive system and did not pose a threat to anyone, a Russian defence ministry spokesman said, adding that it was unclear why its placement had “caused such a stir among our Western colleagues”.

A day earlier, Washington suspended direct US-Russia talks on a Syria ceasefire – a move US Secretary of State John Kerry blamed on Russia’s rejection of diplomacy in favour of helping the forces of President Bashar al-Assad achieve military gains over opposition fighters.

“Today there was very heavy bombing. More than 16 civilians were killed and more than 32 people were injured,” Ibrahim Abu Leith, spokesman for the Syrian Civil Defence in Aleppo, told Al Jazeera.

“The regime is trying to enter the east from several different points. So far, they’ve only been able to take territory in the northeast, taking the Handarat camp and the Shuqayif area.”

The Civil Defence, also known as the White Helmets, is a volunteer rescue group that operates in rebel-held areas across the country.

READ MORE: Syrian army advances as ‘Aleppo carnage’ sparks fury

Government troops, with Russian air support and Iranian and militia ground forces, launched an assault to retake all of Aleppo last month after a week-old ceasefire agreed to by Washington and Moscow steadily fell apart.

Last week, government forces captured the Handarat Palestinian refugee camp north of Aleppo and the nearby Kindi Hospital, which overlooks a key intersection of vital roads. Government troops also took the central neighborhood of Farafra after pushing forward from the Old City.

“Tanks will not be able to advance easily in the streets,” Aleppo-based activist Baraa al-Halabi told The Associated Press news agency, adding the Handarat camp was only taken because it is in an open area.

The US and other Western countries say Moscow and Damascus are guilty of war crimes for deliberately targeting civilians, hospitals, and aid deliveries to crush the will of more than 275,000 people trapped under the siege of Aleppo. The Syrian and Russian governments say they target only “terrorists”.

Rebels said they inflicted losses on pro-government fighters after hours of clashes on the fringe of Sheikh Saeed district on the southern edge of the city’s rebel-held sector.

“We repelled their attempt to advance in Sheikh Saeed and killed 10 regime fighters and destroyed several vehicles,” said a fighter from the Failaq al-Sham rebel group, who gave his name as Abdullah al-Halabi.

But pro-government media said the Syrian army was pressing ahead.

In the 15 days since the collapse of the ceasefire, the Syrian Observatory said it had documented the deaths of 293 civilians in east Aleppo as a result of air strikes and shelling, including 20 on Tuesday.

It has documented 25 deaths in government-held west Aleppo from rebel shelling during the 15 days.

The White Helmets said the death toll in rebel-held areas was more than 500 in a bombing campaign described as “unprecedented” in its ferocity.

Many hundreds of wounded people have been brought to a handful of functioning hospitals, which are short of supplies and have themselves come under attack.

Doctors Without Borders (MSF) said four of the last eight functioning hospitals in eastern Aleppo had been damaged by air strikes in the past four days.

“Doctors are performing brain and abdominal surgeries … on the floors of the emergency rooms, for lack of available operating theatres,” said Pablo Marco, MSF’s Middle East operations manager.

‘Irresponsible and profoundly ill-advised’

Turkey, a long-time foe of Assad, but which has recently repaired damaged ties with Russia, said on Tuesday that it planned to make a proposal to Moscow and Washington to resurrect last month’s collapsed ceasefire.

Turkish President Recep Tayyip Erdogan could speak by telephone to President Barack Obama and President Vladimir Putin about a proposal to revive the failed ceasefire, according to a spokesman who spoke to Reuters news agency.

The spokesman did not give details of the proposal, but a German official said US, British, French, German and Italian officials would meet in Berlin on Wednesday to discuss the next steps.

At a speech in Brussels on Tuesday, Kerry vowed to continue pushing for peace.

“We are not giving up on the Syrian people and we are not abandoning the pursuit of peace,” he said.

He accused Moscow of turning a blind eye to Syria’s use of poison gas and barrel bombs – oil drums packed with explosives – to kill civilians.

The top US diplomat called Russia’s decision to “associate its interests and its reputation with that of Assad” as “irresponsible and profoundly ill-advised”.

Russian Foreign Minister Sergey Lavrov was quoted by Russian news agencies as saying that Moscow would make efforts to resolve the crisis despite the US suspension of talks.