The airstrike on the IS-held village of Baghouz comes as the offensive against the extremists by U.S.-backed fighters of the Kurdish-led Syrian Democratic Forces has intensified over the past few weeks. It also comes as the United States' begins the process of withdrawal from Syria.
Thousands of civilians have fled from the area near the Iraqi border recently as IS has steadily lost ground.
State news agency SANA said 20 people were killed in the airstrike on Baghouz, while the Britain-based Syrian Observatory for Human Rights said 23 people were killed. It said 10 IS members were among the dead.
Both blamed the U.S.-led coalition that has been providing air cover to the SDF in their monthslong offensive to capture the area from the extremists.
"Most of those killed were Syrians displaced from nearby areas," said Europe-based activist Omar Abu Layla of the DeirEzzor 24 monitoring group. He added that the dead included three families who were trying to flee the IS-held area to districts controlled by SDF.
Graham was also expected to speak with Erdogan and other Turkish officials about a proposal for the creation of a "safe zone" in northeast Syria.
The visit by Graham, who has a prominent voice on U.S. foreign affairs, comes days after a suicide bombing, claimed by IS, killed two U.S. service members and two American civilians in the northeastern town of Manbij.
Graham has said he is concerned that U.S. President Donald Trump's troop withdrawal had emboldened Islamic State militants and created dangerous uncertainty for American allies.
The Pentagon identified three of the four Americans killed in the suicide bomb attack in Manbij — Army Chief Warrant Officer 2 Jonathan R. Farmer, 37, of Boynton Beach, Florida, who was based at Fort Campbell, Kentucky; Navy Chief Cryptologic Technician (Interpretive) Shannon M. Kent, 35, from upstate New York and based at Fort Meade, Maryland; and civilian Scott A. Wirtz, from St. Louis.
The Pentagon hasn't identified the fourth casualty, a civilian contractor.
In northwest Syria, the observatory also said an explosion outside an office belonging to an al-Qaida-linked group killed another 11 people on Friday, including seven members of the militant group Hayat Tahrir al-Sham, or Levant Liberation Committee. Smart news agency, an activist collective, said 12 people were killed, many of them militants.
Both said the blast occurred on the southern edge of the rebel-held city of Idlib.
A week ago, members of Hayat Tahrir al-Sham took control over wide parts of Idlib province and the surrounding countryside after forcing rival insurgents to accept a deal for a civil administration run by the militants in their areas.