Turkish President Recep Tayyip Erdoğan has long threatened to send troops into northeastern Syria to clear the border region of Syrian Kurdish fighters whom Turkey considers a serious security threat.
Meanwhile Kurdish forces said the Turkish bombardment on Kurdish-controlled targets on Wednesday caused civilian casualties, despite Turkey's claims that its forces were avoiding residential areas.
Turkish Vice President Fuat Oktay said his country wouldn't "bow to threats" in an apparent response to Trump's warning.
Turkey views Kurdish YPG fighters in northeast Syria as terrorists due to their ties to the militant Kurdistan Workers' Party (PKK) which has waged an insurgency inside Turkey since 1984. Ankara has said it plans to create a "safe zone" in order to return millions of refugees to Syrian soil.
The Britain-based Syrian Observatory for Human Rights put the death toll at eight, including women and children.
President Donald Trump on Sunday abruptly announced American troops would step aside ahead of the Turkish push - a shift in US policy that essentially abandoned the Syrian Kurds, longtime USA allies in the fight against the Islamic State group.
Trump called the Turkish assault a "bad idea" and said he did not endorse it.
Mr Trump added that no American soldiers were stationed in the area being invaded.
Iranian President Hassan Rouhani, a close ally of Syrian President Bashar-Al-Assad, told state media that the "presence of the Syrian army" is the "only solution to ensure safety and security in southern Turkey and northern Syria".
He said Australia was using diplomatic channels to monitor the situation to help determine what "possible global responses" could be taken.
This comes days after the U.S. troops backed away from the border area.
There has also been a chorus of global concern, including from United States allies. But world powers fear Turkey's action could intensify the conflict, and runs the risk of Islamic State prisoners escaping from camps amid the chaos. Around 2,000 of them are foreigners coming from 50 different nations who mostly entered Syria illegally from Turkey before 2015.
Thousands of people fled the Syrian town of Ras al Ain toward Hasaka province, held by the Kurdish-led Syrian Democratic Forces.
Stratford said buses of Free Syrian Army fighters were also transported from northern Aleppo in Syria across the border into Turkey on Tuesday.
"There is a 99 percent chance that the offensive will be launched tonight", he said.
"They are extremely alarmed that such a lightweight treatment of this extremely delicate subject could ignite the entire region", Lavrov said.
What about the fight against the Islamic State group?
The Turkish army has been poised to cross into war-ravaged Syria since the surprise withdrawal of United States troops last weekend.
The Syrian Democratic Forces (SDF) who operate in the area are U.S. allies. "There is huge panic among people in the region", Bali said.
After Erdogan announced that the operation, dubbed "Spring of Peace", had started on Wednesday, Graham called on Trump to reassess the USA position "while there is still time".
In the build-up to the expected offensive, Syria had said it was determined to confront any Turkish aggression by all legitimate means.
The defence ministry on Tuesday said "all preparations for the operation" had been completed.
France "strongly condemns" Turkey's offensive in northeast Syria, European Affairs minister Amelie de Montchalin said Wednesday, within minutes of Ankara launching cross-border strikes. Ras al-Ain was one of the places from which USA troops withdrew on Monday.
The Turkish offensive jeopardizes Kurdish-led forces who have been a key US ally in the bloody fight against ISIS. And he's going to get 100 per cent of the blame. "US should never have been in Middle East", President Trump said in a series of morning tweets.
"Wide, deep, and devastating sanctions", he said.
It also called on Kurds in Syria and overseas to protest against Ankara's planned offensive. However, according to the White House, this responsibility will now fall to Turkey.