The United States vowed a response on Thursday as it said it had confirmed another chemical weapons attack by Syrian President Bashar al-Assad's forces, although there were no fatalities.The Assad regime used chlorine on May 19 in Latakia province during its ferocious offensive to take back the last major rebel stronghold in nearby Idlib, Secretary of State Mike Pompeo said."The United States will not allow these attacks to go unchallenged nor will we tolerate those who choose to conceal these atrocities," Pompeo told reporters in New York, where he was taking part in the UN General Assembly."The United States will continue to pressure the insidious Assad regime to end the violence directed at Syrian civilians and participate in the UN-led political process," he said.READ | Syria 'chemical attack': What we knowHe later took part in a meeting on Syria with his counterparts from France, Germany, Saudi Arabia, Jordan and Egypt, who said in a joint statement: "The use of any chemical weapons in Syria shall not be tolerated."InjuriesFour people were injured in the attack and, while there were no deaths, it marked the first known chemical attack in a year and raised fears of further use, said Jim Jeffrey, the US special representative for Syria."We fear that the regime, which has very weak infantry forces, will try to use chemical weapons once again to make up for its inability to seize ground by combat power," Jeffrey told reporters.No independent verification was available of the attack from northwestern Syria, where rights observers say that more than 1 000 people have been killed and 400 000 displaced since the government began its bombardment in April.The US and France had both earlier aired suspicions of a chemical attack but had held off on making a formal determination, saying more research was needed.International investigators say Assad has repeatedly used chemical weapons against civilian targets in his brutal quest to win the civil war, in which more than 370 000 people have died.Former president Barack Obama had called chemical weapons use a red line but ultimately declined military retaliation.Drawing a contrast, US President Donald Trump ordered strikes with 59 cruise missiles in response to a sarin gas attack in April 2017 in the rebel-held Idlib town of Khan Sheikhun.The reprisal evidently did not deter Assad, who enjoys strong support from Russia, which has vetoed UN Security Council bids to rein in Assad and deployed its military to Syria.