The White House said that Trump has been "fully briefed" on the Manbij attack and that officials would continue to monitor the situation.
Vice-President Mike Pence, speaking at the State Department several hours after the US casualty reports, did not mention the bombing or the deaths, but instead hailed Trump's leadership in combatting the militants in Syria.
"We are bringing our troops home," Pence said in an address to 180 US ambassadors and chiefs of missions abroad gathered for a conference in Washington. "The caliphate has crumbled, and Isis has been defeated."
Surveillance camera video, apparently from the site of the attack, showed the explosion erupting on a busy footpath, sending a child running from the flames with hands clasped over his ears. Bodies and blood trails could be seen spread across the ground in photographs from the immediate aftermath.
The Britain-based Syrian Observatory for Human Rights said at least 19 people were wounded or killed. A Kurdish news agency reported that at least 10 people were injured in the attack, which it said occurred outside a popular restaurant.
Trump announced last month that about 2000 US military personnel would be leaving Syria, almost four years after they intervened as part of an international coalition to dislodge Isis from its self-declared "caliphate" straddling Syria and Iraq.
In public pronouncements, Trump initially justified the decision by saying that Isis had been defeated. Advisers then launched a weeks-long lobbying effort in an attempt to get Trump to change his mind or slow the withdrawal, and defence officials have reminded the President that Isis continues to wage stiff resistance in eastern Syria, while maintaining the ability to launch strikes elsewhere in the country.
The US military announced last weekend that it has begun withdrawing equipment from Syria but declined to provide details, leaving it unclear whether any material has been pulled out of the Manbij area. Pentagon officials said then that initial withdrawals would be limited to equipment and that no troops had yet departed.
The drawdown is widely seen as likely to spark battles for control of areas vacated by the Americans, but there was no immediate indication of any link between the withdrawal plan and today's attack in Manbij.