Turkey and Europe's three most powerful countries have agreed that humanitarian aid must be allowed into north-east Syria and on a UN assessment of the region, the British government said.
They condemned attacks any civilians in Syria, including those in the besieged rebel stronghold of Idlib.
Leaders of the UK, France, Germany and Turkey repeated their support UN-led efforts in Libya towards a "Libyan-owned" political solution in the conflict-ridden country.
"On Syria, the leaders agreed that humanitarian access, including cross-border, must be ensured and that a UN needs assessment should form the basis for getting aid to those who require it in the north-east," a Downing Street spokesman said.
"The leaders said they would work to create the conditions for the safe, voluntary and sustainable return of refugees and that the fight must be continued against terrorism in all its forms.
"They also agreed that all attacks against civilians in Syria, including those in Idlib, must stop."
The statement came after talks between French President Emmanuel Macron, Turkey's Recep Tayyip Erdogan, British Prime Minister Boris Johnson and German Chancellor Angela Merkel.
Overnight, Britain's chief representative at the UN, Karen Pierce, welcomed UN Security Council support for Ghassan Salame, who leads the UN's special mission to Libya.
The UNSC urged countries to abide by a strict arms embargo in Libya. Among those accused of breaching it is Turkey, which has faced censure for a maritime agreement it signed with the UN-backed government in Tripoli.
Mr Macron earlier criticised Turkey over its recent incursion into north-east Syria and for attacking local groups with whom countries including France were allies against ISIS. He claimed that Turkey was backing "ISIS proxies".
Mr Erdogan earlier criticised Mr Macron for comments describing Nato as brain dead.
After the Turkish president met the three European powers in London’s Downing Street, Mr Macron said talks had been “very useful” but not “all the ambiguities” were resolved.
Turkey regards the Kurdish YPG militia in north-east Syria as a terrorist group.
Many western countries fought alongside a YPG-led force to remove ISIS from large parts of Syria.
After the meeting, Mr Macron said gaps in understanding over the Kurdish force remained and had done for a long time.
Mrs Merkel struck a more positive tone, saying the talks with Turkey had been “good and sensible”.
“Notwithstanding all the differences, I’m going into the meeting relatively optimistically,” she said, before the main Nato summit in London on Wednesday.
Moments before heading to a reception at Buckingham Palace, she joked: “The time for that was too short. And obviously the Queen cannot be kept waiting.”
Mr Erdogan told Turkey's state-run news agency Anadolu that the talks were "good".