US Secretary of State Mike Pompeo has denied Washington gave Turkey the go-ahead for its offensive in Syria.
Turkey had on Wednesday bombed parts of north-eastern Syria in order to create a safe zone cleared of Kurdish militias which will also house two million Syrian refugees.
In an earlier statement, Trump said the US did not endorse this attack, calling the operation a bad idea.
Trump also said Turkey would be responsible for ensuring that suspected IS fighters being held captive remained in prison and that IS did not regroup.
Turkish armed forces tweeted they had struck 181 terrorist targets as part of what has been dubbed Operation Peace Spring. The Syrian Democratic Forces said one of the prisons holding IS fighters was hit by the airstrike.
Several towns and villages were hit in the assault, forcing thousands of civilians to flee their homes.
The Kurdish Red Crescent said at least seven civilians had so far been killed, two of them children, and at least 19 more critically injured including four children.
Meanwhile, Kurdish authorities have called for a general mobilization and urged people to head to the border with Turkey to resist the offensive.