The US 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, US 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," Mr Pompeo said in New York, on the sidelines of 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 later took part in a meeting on Syria with foreign ministers from France, Germany, Saudi Arabia, Jordan and Egypt. "The use of any chemical weapons in Syria shall not be tolerated," the ministers said in a joint statement. Four 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," Mr Jeffrey said. No independent verification was available of the attack from north-west Syria, where rights observers say 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 earlier told of their suspicions of a chemical attack but held off making a formal determination, saying more research was needed. International investigators say the Syrian regime has repeatedly used chemical weapons against civilian targets in the civil war. Former president Barack Obama had called chemical weapons use a red line but ultimately declined military retaliation. Drawing a contrast, 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 Sheikhoun. Mr Pompeo said that the US was donating an additional $4.5 million (Dh16.5m) to the Organisation for the Prohibition of Chemical Weapons, the body in Europe that monitors the such arms. He appeared to downplay the prospect of military action, saying the attack involved chlorine, which affects the respiratory system. The Khan Sheikhoun attacks, which the UN said killed 83 people, used sarin, an ultra-potent gas that devastates the nervous system. "So it's a bit of a different situation," Mr Pompeo said. But "the Syrian regime should know and the world should appreciate the fact that we're going to do everything we can reasonably do to prevent this kind of thing from happening again".