Russian Federation launched a military intervention in support of Assad in 2015, helping his forces to reclaim large parts of the country from opposition fighters and jihadists.
The opposition fired artillery at government forces, the Syrian Observatory for Human Rights also said.
"We are seeing the increased attacks by the regime, especially targeting hospitals, schools and civilians recently in Idlib", he said.
Turkey hopes that Trump and Erdogan's personal rapport can help it skirt sanctions Congress plans to implement over the S-400s, even though the American leader lacks the authority to veto the measures created to dissuade countries from doing business with Russian defense and intelligence entities.
Tensions between the United States government and Turkey over the S-400 have been simmering for months but have taken on a new urgency as Ankara gears up to receive its first deliveries of the system this summer.
"We will do what is necessary because we want peace to be sovereign here, for the deaths to stop", he told a press conference in Istanbul.
"If the United States takes any negative actions towards us, we will also take reciprocal steps", Mevlut Cavusoglu, the foreign minister, said.
Back in 2017, Turkey agreed to purchase Russia's advanced S-400 air defense missile system over the strong objections of both the U.S. and North Atlantic Treaty Organisation, of which Turkey is a member.
Cavusoglu insisted moderate Syrian rebels in Idlib - for which Turkey acts as guarantor in peace negotiations - had stuck to their obligations under a buffer zone agreement reached last September. The details of the ceasefire, including how long it would last, were not revealed and remain unclear, but the regime's continued shelling of Idlib and direct attack on the Turkish outpost has effectively jeopardised the short-lived ceasefire. Nevertheless, Assad's forces had started there at the beginning of may a ground offensive.
"Turkey is not just an F-35 customer".
It appears that Mr Erdogan is hoping to avoid the damaging repercussions of the Russian agreement by appealing to US President Donald Trump personally, first by telephone and later with a meeting at the G20 summit in Japan in two weeks. The American point of view is either you take [the S-400s] or you don't.
It said Russian authorities were contacted over the incident.