In a recent discussion about Syria, people familiar with the exchange said, President Trump threatened to conduct a massive attack against Mr. Assad if he carries out a massacre in Idlib, the northwestern province that has become the last refuge for more than three million people and as many as 70,000 opposition fighters that the regime considers to be terrorists.
The Wall Street Journal was the first to report the decision, citing US officials.
"The global community has made it Turkey's responsibility to get rid of the extremists of Hayat Tahrir al-Sham without providing any help", Muhittin Ataman, director of foreign policy studies at Ankara-based think tank SETA, told a conference last week.
The preparations came as Russian jets and Syrian helicopters on Monday resumed intensive strikes in Idlib and Hama for a third day, with sources saying the Syrian army helicopters were dropping barrel bombs.
"A regime assault [in Idlib] would also create serious humanitarian and security risks for Turkey, the rest of Europe and beyond", he added.
On Monday, a senior Turkish member of parliament said that the government in Ankara had no plans to open the border wall to allow Syrian refugees to cross into Turkey.
Erdogan said despite being the target of terror groups Daesh and the PKK, Turkey did not lose its resolve to help Syrians. It wasn't clear from the latest intelligence if Mr. Assad also had given the military permission to use sarin gas, the deadly nerve agent used several times in previous regime attacks on rebel-held areas.
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"In an attempt to prevent the assault, my government contributed to the creation of a deconfliction zone and set up 12 observation posts to document and report ceasefire violations", he said.
In the Commons, the United Kingdom foreign office minister Alistair Burt refused to give an undertaking that MPs will be given a vote if the United Kingdom sees a need to join the U.S. in mounting reprisal raids for what it adjudges to have been a chemical attack by the Syrian government. But the Pentagon pushed for a more measured response, US officials said, and the idea was eventually rejected as too risky.
Turkey massed tanks and commandos at its frontier and accelerated work to cope with a possible surge of refugees after Russian Federation refused to call off an offensive against the last rebel bastion in Syria. "In order to eliminate terrorist and extremist elements in Idlib and to bring to justice foreign fighters, a more comprehensive worldwide counterterrorism operation is necessary", he said. Idlib is the last redoubt of terrorist groups including Jabhat al-Nusra and some offshoots of Al Qaeda.
"Not only innocent Syrians, but the entire world stands to pay the price [otherwise]", he said.
"Turkey has done everything in its power to stop the bloodshed next door".
Syrian forces, backed by Russian Federation and Iran, have been massing for weeks around Idlib's borders where the last bastion of Islamist-dominated rebel groups is seeking refuge and the final obstacle for Assad to declare victory.
"There needs to be ways of dealing with this problem that don't turn the next few months in Idlib into the worst humanitarian catastrophe with the biggest loss of life in the 21st century", Lowcock told reporters.
Airstrikes pounded southern Idlib and northern Hama over the weekend.
Turkish President Recep Tayyip Erdogan has urged a ceasefire be agreed to avert a "bloodbath" in Idlib.