BEIRUT — Warplanes on Saturday struck the Syrian town where a chemical attack had killed scores of people earlier this week, as Turkey warned that a retaliatory US missile strike on a Syrian air base would only be ‘‘cosmetic’’ if greater efforts are not made to remove President Bashar Assad from power.

The airstrikes on the opposition-held northern town of Khan Sheikhoun, where 87 people were killed in the chemical attack earlier this week, killed a woman and wounded her son, according to the Britain-based Syrian Observatory for Human Rights and the Local Coordination Committees, an activist collective.

Elsewhere in Syria, US-led airstrikes killed at least 21 people, including a woman and her six children who were fleeing on a boat across the Euphrates River near the Islamic State group’s self-styled capital, Raqqa, the target of a major offensive by US-backed and Kurdish-led Syrian forces, activists said.

Near the central city of Homs, a bomb exploded aboard a bus carrying workers, killing a woman and wounding more than 20, according to state TV and the Observatory.

The chemical attack prompted the US to launch 59 Tomahawk missiles at a Syrian air base early Friday, killing nine people and marking the first time Washington has directly targeted Syrian government forces since the war began in 2011.

The move was welcomed by the Syrian opposition and its main backers, including Turkey and Saudi Arabia, but harshly condemned by Russia and Iran, which back Assad and said striking his forces would complicate the struggle against extremist groups.

Turkish Foreign Minister Mevlut Cavusoglu said the US strike should be the start of a renewed effort to end the civil war, which has killed an estimated 400,000 people and displaced half of Syria’s population.

‘‘If this intervention is limited only to an air base, if it does not continue and if we don’t remove the regime from heading Syria, then this would remain a cosmetic intervention,’’ he said.

He said the best outcome would be a peace agreement that leads to a transitional government accepted by all Syrians, followed by elections in which all Syrians, including those living abroad, could vote for new leadership.

For that to happen, he said, ‘‘this oppressive Assad needs to go.’’

Iran, which has provided crucial military and political support to Assad, meanwhile called for a fact-finding mission to determine what caused the chemical attack in Khan Sheikhoun. State television quoted Iranian President Hassan Rouhani as saying the committee should be impartial and ‘‘must not be headed by Americans.’’

Rouhani said ‘‘neutral countries should come and assess to make it clear where the chemical weapons came from.’’

Syria’s government has denied carrying out any chemical attack, and Russia’s Defense Ministry said the toxic agents were released when a Syrian airstrike hit a rebel chemical weapons arsenal and munitions factory.