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 U.S. missile strike on a Syrian airbase would only be "cosmetic" if greater efforts are not made to remove President Bashar al-Assad from power.

The airstrikes on the opposition-held northern town of Khan Sheikhoun, 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. It's the same town where 87 people were killed in a chemical attack earlier this week.

Elsewhere in Syria, U.S.-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 U.S.-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 U.S. to launch 59 Tomahawk missiles on a Syrian airbase early Friday, which killed nine people and marked the first time Washington has directly targeted Syrian government forces since the war began in 2011.

A chemical weapons attack in Khan Sheikhoun earlier this week that killed 87 people motivated a retaliatory U.S. missile strike on a Syrian airbase. (Ammar Abdullah/Reuters)

In a letter to Congress on Saturday, U.S. President Donald Trump said the missile strike targeted the source of Tuesday's chemical attack.

"I directed this action in order to degrade the Syrian military's ability to conduct further chemical weapons attacks and to dissuade the Syrian regime from using or proliferating chemical weapons," he wrote in the letter.

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 U.S. 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.

'Cosmetic intervention'

"If this intervention is limited only to an airbase, 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 Defence Ministry said the toxic agents were released when a Syrian airstrike hit a rebel chemical weapons arsenal and munitions factory.

British Foreign Secretary Boris Johnson cancelled a planned trip to Russia because of fast-moving events in Syria. Johnson said the situation in Syria has changed "fundamentally" following the chemical attack and the U.S. response.

Johnson condemned Russia's continued defence of Assad "even after the chemical weapons attack on innocent civilians."

U.S. Secretary of State Rex Tillerson meanwhile plans to meet with G7 foreign ministers in Europe next week before going on to Moscow. Johnson said Tillerson will be able to give a "clear and coordinated message to the Russians."

Syrian protests

People attend a protest against American missile strikes in Syria, in front of the UN building in Damascus, Syria, on Saturday. (Omar Sanadiki/Reuters)

In Damascus, dozens of Syrian students gathered outside the offices of the United Nations to protest the U.S. missile attack.

The protesters in the capital held banners and chanted "Death to America" and "Death to Israel."

One of the banners they carried read: "The Iraqi scenario will not be repeated in Syria." They were referring to the 2003 U.S.-led invasion of Iraq after Washington said Saddam Hussein was hiding weapons of mass destruction -- a belief that later turned out to be incorrect.

University student Ashraf Fadel said he came to denounce "the unjust American aggression against Syria." He added that the United Nations was "created to support America instead of serving the wronged people."