Turkey has launched a military operation against Kurdish fighters in northeast Syria just days after US troops pulled back from the area.
Warplanes and artillery have targeted militia positions in several towns in the border region of Syria.
Turkish President Tayyip Erdogan, announcing the start of the action, said the aim was to eliminate what he called a "terror corridor" on Turkey's southern border.
However European countries immediately called on Turkey to halt the operation.
Thousands of people fled the Syrian town of Ras al Ain towards Hasaka province, held by the Kurdish-led Syrian Democratic Forces.
The Turkish airstrikes killed two civilians and wounded two others, the SDF said on Wednesday.
Turkey had been poised to enter northeast Syria since US troops, who have been fighting with Kurdish-led forces against Islamic State, started to leave in an abrupt policy shift by President Donald Trump.
The withdrawal was criticised in Washington as a betrayal of America's Kurdish allies.
A Turkish security source told Reuters the military offensive, dubbed "Operation Peace Spring", opened with airstrikes.
Turkish howitzer fire then hit bases and ammunition depots of the Kurdish YPG militia.
The artillery strikes, which also targeted YPG gun and sniper positions, were aimed at sites far from residential areas, the source said.
A Reuters cameraman in the Turkish town of Akcakale saw several explosions across the border in the Syrian town of Tel Abyad, where a witness reported people fleeing en masse.
Large explosions also rocked Ras al Ain, just across the border from the Turkish town of Ceylanpinar, a CNN Turk reporter said.
The sound of planes could be heard above and smoke was rising from buildings in Ras al Ain, he said.
The SDF said military positions and civilians in the city of Qamishli and the town of Ain Issa - more than 30km inside Syria - had been hit, and said there were initial reports of civilian casualties.
Turkish media said mortar and rocket fire from Syria struck the Turkish border towns of Ceylanpinar and Nusaybin. There were no immediate reports of casualties there.
Amid deepening humanitarian concerns, Germany said Turkey's action would lead to further instability and could strengthen Islamic State.
European Commission President Jean-Claude Juncker called on Turkey to halt the military operation.
Juncker said the bloc would not fund Ankara's plans in the region.
"If the plan involves the creation of a so-called safe zone, don't expect the EU to pay for any of it," he told the EU parliament.
Kurdish-led forces have denounced the US policy shift as a "stab in the back".
Trump denied he had abandoned the forces, the most capable US partners in fighting ISIS.