United Nations Secretary-General Ban Ki-Moon expressed his concerned Monday about “the weakening taboo” against chemical weapons attacks, after numerous reports that more such attacks have taken place in Syria in recent weeks.

In a letter to the UN Security Council, Ban expressed fears that the use of chemical weapons could become “normalized in this or any conflict, present or future” and that “It is imperative that those responsible for the use of chemical weapons should be held accountable.”

The Organization for the Prohibition of Chemical Weapons (OPCW) reported four instances of chemical weapons use in Syria between September 23 and October 22. The OPCW’s mission prevents it from assigning responsibility for the attacks, but the Security Council’s Joint Investigative Mechanism, put in place in 2014, has officially determined that the Syrian government had used chlorine in three attacks and that ISIS had used mustard gas once.

The Assad regime killed hundreds of civilians in a chemical attack on a Damascus suburb in August 2013. Despite a deal drawn up in the wake of this attack to rid Syria of its chemical weapon stockpile, the Assad regime has continued to use chlorine gas, and traces of chemical weapons were found in May 2015 at facilities that Assad had not declared. Last year, Iran attempted to block diplomatic efforts to condemn Syria for its violation of the chemical weapons agreement it had signed.

Assad prevented inspectors from accessing all of his chemical weapon stores in July 2015, allowing Syria to avoid complying with the 2013 deal it made with the U.S. and Russia to destroy its chemical weapons stockpiles. American intelligence at the time indicated that Assad still retained “caches of even deadlier nerve agents,” The Wall Street Journal has reported. OPCW Director-General Ahmet Uzumcu has expressed doubt that Syria had been accurate in its disclosure of its chemical weapons facilities.

Ely Karmon, a senior research scholar at Israel’s International Institute for Counter-Terrorism, warned that the failure to fully rid Syria of chemical weapons boded poorly for the success of the nuclear deal with Iran. “If this is the record of the international community in dismantling and monitoring the chemical and nuclear facilities in Syria,” he asked, “how exactly it will do better in monitoring the vast Iranian nuclear infrastructure?”

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